It's like most other hobbies - collect them and gather dust. I have probably a dozen or so that I open at least once a year - most are go-to's for specific cuisines, like Thai, Italian, etc. Some, like Alton Brown's, are just a fun read. If I'm looking for a specific recipe, I'll dig through my books first, and if I don't find what I'm looking for (very rare), then I'll turn to the Internet.