If you happen to be in Wisconsin, a copy of Zimmerman's original treatment (which is what Tolkien critiques in Letter 210), and production notes, is part of the Marquette University Tolkien collection, in Milwaukee. The full Letter 210 is also part of that collection; by Humphrey Carpenter's own admission only a portion of Tolkien's criticism was actually published in The Letters of J.R.R. Tolkien:

Aside from the (pretty hilarious) Letter 210, and some others, there's been some (literary) criticism of the treatment, and of Tolkien's response; most notable is an essay by American author Janet Brennan Croft, who explores several early attempts at bringing The Lord of the Rings to film, including Zimmerman's effort; she writes (among other things):

• The production notes indicate that the producers planned to use a mix of animation, miniature work, and live action, and to make a three hour film with two intermissions

• Gandalf hypnotizes and psychically frog-marches the eavesdropping Sam into Frodo's study

• The company is attacked at the Gates of Moria by wolves, which Gandalf dispatches with a few lightning bolts, and in Moria he magically opens a chasm to swallow up the attacking orcs. During Denethor's suicide scene, Gandalf levitates the body of Faramir from the pyre. In a final act of wizardry, he turns the Ringwraiths to stone one by one at the Battle of the Black Gate while the assembled armies watch in silence.

• [S]everal armed attacks on Strider and the Hobbits as they flee from Weathertop to Rivendell, and sending them over Rauros Falls in their flimsy rowboats.

• Sam actually abandons Frodo to Shelob and carries the Ring to Mount Doom himself. He realizes Frodo is still alive, but his duty to Middle-earth triumphs

• At the Cracks of Doom he is about to toss the Ring into the fire when he is attacked by a crazed Frodo, who in turn is attacked by Gollum with no indication of where either of them has been hiding since Shelob's lair. The weakly written ending has Frodo awakening in Minas Tirith after Aragorn's wedding, and immediately sailing away with the Elves.

• Annoying spelling errors are repeated throughout. The entire Treebeard sequence and the meeting with Faramir are both truncated to the point of unintelligibility. The intercutting of the separate story lines of The Two Towers and The Return of the King is disorienting, switching from Mount Doom to the Black Gate every few seconds at the climax.

• Here's how each script handles Bilbo handing the Ring over to Gandalf after the party. The Bakshi film follows the book fairly closely, with Bilbo sealing the Ring in an envelope, and Gandalf catching the envelope as he drops it. Boorman, as expected, does his own thing and has Bilbo drop it in Gandalf's hat. But Zimmerman and Jackson both use the opportunity to do something more cinematically interesting - in these versions, Bilbo drops the Ring on the floor and Gandalf refuses to touch it, leaving it for Frodo to pick up. Not only is the Ring a more obvious and visible menace, it allows the director to visually echo Bilbo picking up the Ring in Gollum's cave.

• The Zimmerman treatment vastly reduces [female characters'] importance, cutting Galadriel's temptation, bringing Arwen onscreen only for her wedding, and dropping Eowyn's attraction to Aragorn.

Porter comments that ‘Zimmerman’s script features caricatures of Merry and Pippin as pesky younger cousins, without differentiating between the hobbits or offering any depth to their characterisation’, and that later screenwriters also ‘often presented one-dimensional hobbits instead of Tolkien’s more complex characters’ [...] ‘Their planning and forethought, as well as their steadfast loyalty to Frodo, are greatly diminished when they merely follow Frodo on a whim, instead of [after] months of planning to accompany their friend and cousin’

If you happen to be in Wisconsin, several documents related to the film are part of the Marquette University Tolkien collection, in Milwaukee:

• Zimmerman's treatment, with Tolkien's annotations
• Production notes for the unmade film, written and annotated by Zimmerman
• Some letters between Zimmerman and Rayner Unwin, son of Tolkien's publisher Stanley Unwin
• The full Letter 210, where Tolkien rips into Zimmerman's treatment. It's notable that, as far as I can tell, this is the full letter; by Humphrey Carpenter's own admission, the version published in Letters is only a selection of Tolkien's full commentary:

Aside from those, there's been some (literary) criticism of the treatment and of Tolkien's response. Most notable is an essay by American author Janet Brennan Croft, who explores several early attempts at bringing The Lord of the Rings to film, including Zimmerman's effort; she writes (among other things):

• The production notes indicate that the producers planned to use a mix of animation, miniature work, and live action, and to make a three hour film with two intermissions

• Gandalf hypnotizes and psychically frog-marches the eavesdropping Sam into Frodo's study

• The company is attacked at the Gates of Moria by wolves, which Gandalf dispatches with a few lightning bolts, and in Moria he magically opens a chasm to swallow up the attacking orcs. During Denethor's suicide scene, Gandalf levitates the body of Faramir from the pyre. In a final act of wizardry, he turns the Ringwraiths to stone one by one at the Battle of the Black Gate while the assembled armies watch in silence.

• [S]everal armed attacks on Strider and the Hobbits as they flee from Weathertop to Rivendell, and sending them over Rauros Falls in their flimsy rowboats.

• Sam actually abandons Frodo to Shelob and carries the Ring to Mount Doom himself. He realizes Frodo is still alive, but his duty to Middle-earth triumphs

• At the Cracks of Doom [Sam] is about to toss the Ring into the fire when he is attacked by a crazed Frodo, who in turn is attacked by Gollum with no indication of where either of them has been hiding since Shelob's lair. The weakly written ending has Frodo awakening in Minas Tirith after Aragorn's wedding, and immediately sailing away with the Elves.

• Annoying spelling errors are repeated throughout. The entire Treebeard sequence and the meeting with Faramir are both truncated to the point of unintelligibility. The intercutting of the separate story lines of The Two Towers and The Return of the King is disorienting, switching from Mount Doom to the Black Gate every few seconds at the climax.

• Here's how each script handles Bilbo handing the Ring over to Gandalf after the party. The Bakshi film follows the book fairly closely, with Bilbo sealing the Ring in an envelope, and Gandalf catching the envelope as he drops it. Boorman, as expected, does his own thing and has Bilbo drop it in Gandalf's hat. But Zimmerman and Jackson both use the opportunity to do something more cinematically interesting - in these versions, Bilbo drops the Ring on the floor and Gandalf refuses to touch it, leaving it for Frodo to pick up. Not only is the Ring a more obvious and visible menace, it allows the director to visually echo Bilbo picking up the Ring in Gollum's cave.

• The Zimmerman treatment vastly reduces [female characters'] importance, cutting Galadriel's temptation, bringing Arwen onscreen only for her wedding, and dropping Eowyn's attraction to Aragorn.

Porter comments that ‘Zimmerman’s script features caricatures of Merry and Pippin as pesky younger cousins, without differentiating between the hobbits or offering any depth to their characterisation’, and that later screenwriters also ‘often presented one-dimensional hobbits instead of Tolkien’s more complex characters’ [...] ‘Their planning and forethought, as well as their steadfast loyalty to Frodo, are greatly diminished when they merely follow Frodo on a whim, instead of [after] months of planning to accompany their friend and cousin’

Also worth a read, as mentioned earlier, is Letter 210, which contains a selection of Tolkien's commentary on the treatment. It's too long to quote here in its entirety, which is sad because it really is quite funny. Tolkien Gateway has a lengthy summary, if you're so inclined, but I just want to quote a single passage (emphasis his):

Part III.... is totally unacceptable to me, as a whole and in detail. If it is meant as notes only for a section of something like the pictorial length of I and II, then in the filling out it must be brought into relation with the book, and its gross alterations of that corrected. If it is meant to represent only a kind of short finale, then all I can say is: The Lord of the Rings cannot be garbled like that.

^{The Letters of J.R.R. Tolkien 210: To Forrest J. Ackerman. June 1958}