[SE sites don't really like "any other suggestions?" questions, but we can probably discuss specific design aspects for this specific project in order to make "good" answers. If this gets voted closed, so be it.]
This is definitely one of those designs where form follows function. If we want to consider this as serious fun, let's over-build a little library for fun and profit. Consider this one of those situations where any fun design will be in the details, and stick to the absolute basics for material and build.
This is basically a box, but one that has to withstand years unprotected by the elements. We can't even bury part of it in the ground and put trees around it to help. It's stuck on a pole or fence, and the openings have to satisfy how people want to use it, not how most of the wet will be driven in. So, it has to be simple, but it also has to take advantage of your material and design to minimize these major problems.
Think of this not as a little house, but a little garage or shed (or ridiculously overbuilt birdhouse). You do not need "rafters" because your roof just doesn't have that much mass. There is no way the walls will bow out because of a snow-load, for example. So, make an oversized roof to help protect the walls and the openings tied directly to the walls. Think about those house designs that we see in monsoon-prone areas.
We also don't have to worry about making anything airtight. In fact, like buildings that live through monsoons, we want as much air movement a possible.
Consider how water seeks any seam, collects and then seeks more seams. Reducing water ingress is actually a number of interconnected ideas:
- Slough off water as much as possible. Don't expose seams, and assume water will be driven in from all sorts of directions. Make sure you give it plenty of opportunity to drain full away.
- Assume water will enter, and allow for that ingress to drain off as well; we don't want overlapping seams, for example, but we also want water to drain out of places it has leaked into. Standing water is the enemy.
- Make sure openings (i.e. any cute doors you design) have simple drip-edges and ways for driven moisture to drain out the bottom and to the outside. Butt-join doors on little libraries always leak into the main area!
- Assume water will enter the main "living area", and the contents might get damp. Don't let the contents sit in damp, and give it space to breathe and dry out.
With all this in mind:
- Oversized high-pitch roof to protect walls and openings. Roof doesn't expose a seam at the apex, but rather uses two or more layers so seams are staggered. Make sure moisture can run along the layers to the outside edges.
- For air flow I'd design in soffits (really!) to let heat and moisture escape. See the bullet point about pest control, though.
- Don't bother with a ceiling or rafters. We want this airy with lots of space to the roof-line. It's not like we are going to blow insulation into the attic, so creating an attic space is useless.
- Main area should actually have a floor and sub-floor. I'd consider staggered drain holes in the floors directly to the outside bottom, and slope the floor for complete drainage. Standing water is the enemy here.
- Doors and openings should include drip edges and allow drainage to the outside. If there is no door (because little doors always sag and become unusable) I'd bring the roof out, and/or design a little porch to protect the opening.
- For material I'd definitely spring for pressure treated lumber for the main build, and get some scraps of prettier weather resistant stuff for the outside (e.g., Western Red Cedar if you live in the Americas). Or stick to PTL for the finish as well. (If you want to use regular lumber and good paint, you'll be repainting it regularly. Just saying.)
- Don't forget about pests and mold. Any library in a temperate zone is going to eventually attract arthropod pests and allow for fungal growth. The former can be somewhat alleviated by the use of screens and sealing seams (this is where pest control and moisture control overlap). You might want to manage fungal growth with the appropriate finish on the inside.
And that's one way to over-engineer a little library.
