I'm the developer of Books, and I've seen a number of comments suggesting the Booxter "borrowed" features from Books. Personally, I have no problem with Booxter borrowing ideas and implementing them in its product. (I would have some issue with code being "borrowed", but ideas are not code.)

For what it is worth, I originally wrote Books to fill a need that I didn't see being addressed in the OS X community. So, rather than buy Library (The 1.x version didn't have the features I needed.), I rolled my own application. Had Library or Booxter been at the state they are now, I would have probably paid them their VERY reasonable prices, and done something else besides developing Books.

I'm meandering here, but I would like to echo the words of another poster. If Booxter (or Library) suits your needs and you feel that it is worth the $15, by all means, send them the cash. The more OS X software being developed, the better. If you like Books instead, download it.

And for what it is worth, the release of Booxter did light a fire under my ass to make Books better. So, while Booxter may have borrowed some ideas from Books, I saw some things that were done better in Booxter and attempted to bring Books up to par. So, I imagine that there was plenty of give and take from both parties.