What you got is all there is in W20!
The Changing Breeds always had been the "offshoot" of the Werewolf game, and the one book is almost all they got in W20. The best information you get is from older editions. However, there's a reason you don't get hits in the core book or most other books: Den Realms are an inherently Bastet thing. There are other pocket realms that appear in other types, such as a Nagah-nest's common moveable Umbral Glade or an Ananasi's room-sized Umbral Refuge, but those are also mostly forgotten in W20.
In general, Werebeasts but for Garou had often little to no information in the books that don't directly deal with them.
Den Realms in Publication History
1st Edition
Den Realms were first introduced in the 1st Edition Werewolf Players Guide. That's also where Bastet first appear.1 The rules presented there are wholly obsolete though.
2nd Edition
The whole concept and rules for the Den Realms were rewritten and made more streamlined for Bastet: The Player's Guide to Werecats for Werewolf the Apocalypse, Changing Breed Book 1 which is 2nd edition. What a title, yes? Most call it "Breed Book Bastet" for a reason.
It is a full-page section in chapter 3 explaining what is a Den-Realm, how the owner can use it, that it makes it harder for anyone not allowed by the owner to pass to the umbra, and that it is one of the few ways most Bastet below rank 4 access the umbra. There are some other implications, like the land having a feedback loop to the den realm and the owner. The section also describes how the realms are passed down or claimed in the social context. The mechanics for taking ownership are found in Chapter 4 with "Rite of Claiming"2
Revised Edition
Players Guide to the Changing Breed for Revised Edition reprints a good chunk of that section almost unaltered, mostly accounting for Nagah who got their own version of it in their own Breed Book.3 The Revised edition update was to a good part a "rules update" and less of a background book.
W20
For W20, the reprint is again in the Changing Breeds book, this time only as a half-page paragraph summary. As a result, most of the rules never got a facelift to W20, and you will have to be inspired by older text.4
While W20 Umbra mentions a single Den-Realm, it is when it claims that The Abyss used to be a Den-Realm.5
The Anthology Songs of Sun and Moon: Tales of the Changing Breeds has one story that in part plays at a Den-Realm.6
Other Pocket Realms
Nagah
Nagah gain the Ananta background since their Breed Book. This is different form a den realm in many aspects: where a den realm is static, Anata are moveable as a marble. Where a den realm is personal, Anata are shared. Where a den realm is connected to the land and influences it back and forth Anata are isolated pockets. Where a den realm blocks others from the Umbra, Anata are a bubble in the Umbra that just... sits there and because of the tiny size don't block people really: the largest den realms can be as large as a small county, while the biggest Anata is only about as big as the smallest den Realm. And they luckily provide a gift to create and bind one to a group of Nagah.7
Ananasi
Ananasi get Sylie (SI-lee), their secret, tiny and personal tiny pocket in the Umbra. It's all different from other breeds, because Ananasi are pretty weaver-aligned and... it's complicated. They get the Rite of Spinning to make and remake their little umbral dens again whenever they want.8
How to make one?
The Rite of Caern Building is a Level 5 Caern Rite, and creates a full on new Caern - it is clearly overkill - but it gives us a strong hint that the rite to make a den realm is less powerful, so it is not a level 5 one. So, let's look at them in order:
Rite of Spinning is a level 1 rite and got a W20 Reprint. It's tiny and expensive in Gnosis.
Birthing the Anata is a level 2 one with preconditions and experience cost, as it has to be done in the umbra - so pretty much inside someone's Anata. It's also only creating a relatively tiny pocket, so... 2 seems appropriate.
On the other hand, Rite of Claiming is a level 3 rite and is to take ownership. However, it also can be used to make a new one, if you read the one sentence that notes so carefully:
This mystic secret proclaims the foundation, or transferral of
a Den-Realm.
Otherwise, it is depicted in the Bastet Breed Book to just be "a standard roll", missing the notion of the experience costs for the background. Just an oversight I assume.
1 - The Werewolf Player's Guide: A Complete Players Handbook for Werewolf the Apocalypse, Stone Mountain (1993), p.154 (stepping sideways), p.157.
2 - Bastet: The Player's Guide to Werecats for Werewolf the Apocalypse, Changing Breed Book 1, Clarkston (1997/1998), p.26, 83-84, 121.
3 - Players Guide to the Changing Breeds: A Sourcebook for Werewolf the Apocalypse, Stone Mountain (2003), p.160-162.
4 - Changing Breeds: Werewolf the Apocalypse 20th Anniversary Edition, s.l.(2013), p.210.
5 - Umbra the Velvet Shadow, 20th Anniversary Edition, s.l.(2014), p.43.
6 - Songs of Sun and Moon: Tales of the Changing Breeds, s.l.(2014), p.22
7 - Nagah: A Sourcebook for Werewolf the Apocalypse, Changing Breed Book 9, Clarkston (2001), p.71-72, 92.
8 - Ananasi: A Sourcebook for Werewolf the Apocalypse, Changing Breed Book 7, Clarkston 2000), p.15, p.41-43, p.96.
