People have covered a lot of good stuff regarding what I look for, but specifically:
Organisation. How easy is it to find information within the book? Will it still be easy if I'm trying to do it mid-session, with the players asking questions and making irritatingly high perception checks? Ideally you ought to be able to read the text in full once, comprehend it, then be able to get the most important bits of it from a quick glance back mid-sesh. I recently read the remastered edition of Deep Carbon Observatory, and found it to be my platonic ideal of a well-organised adventure; it's got clear headings, oversized page numbers, and constantly cross-references itself so you're never in doubt as to where to turn for what you're looking for.