Ok, wow - this is awesome. Manybooks.net is a bit Project Gutenberg (they have a lot of books, many of which are in the public domain), but they have a whole slew of download options for the books (which I assume they build on the fly, which is cool).

The best part is that they have an option to take any of their texts, and build a custom PDF. You can specify the width and height of the pdf to be made, the font size, and the line spacing. I'm going to try a few settings to see what my optimal PDF sizes are. 90mm x 120mm seems to be the right size, but 12pt font and 1.25 line spacing is a bit too much, I think. Also, my margins are a bit much.