They say the best way to learn something is to teach it. Well, I say writing a book sure counts as teaching because writing the 2nd edition of Lucene in Action sure has taught me all sorts of juicy details about Lucene, far more than I would have learned on my own.
I can also say that writing a book about an active open-source project is very demanding. I try to keep the manuscript current, as changes are happening to Lucene, but then more than once I've been burned by keeping it just a little too current, only to see that the community up and changed its mind on something I had already folded into the book's manuscript and source code!
Finally, as Lucene is getting very close to releasing 2.9 I'm now scrambling to fix the loooong tail of little things all throughout the book. Even once I finish that, it's several more months for a deep technical review, Manning's production process, etc. I'm looking forward to finishing!