Imo it's just not worth it, if somebody can read it he can make a copy of it and redistribute it "illegally". Only thing you'll accomplish is pissing of the ones who retrieve it legally and the "harder" (harder for the average Joe) you make it the more annoying it'll be.
Only reliable "secure" way of publishing an ebook is not publishing it as an ebook at all.
Performance wise they seem pretty similar (althoug I couldn't find any up to date benchmarks), feature wise it depends on what you are looking for. Sphinx has aggregate support and lucene can update the index (sphinx can "only" merge deltas).
If you don't use java already I would always go for sphinx, java adds allot of dependencies/"things you have to take care of".
One neat feature added in this release is that you can connect to sphinx using any mysql client. So there is no need for a specialized client api and you can use it with everything (literally).
Same thing here, if you do search on your website you should have a look a sphinx. Just because it's so insanely fast (even if you don't do full text search). Once you start using it you'll never look back ;).