Thanks for detailed answers.
@baf,
@KevinH, I fully appreciate your work and understand your reasons and exclusive right for choosing a license for your code. I endorse (L)GPL in general, but in case of unique closed-source libraries (I mean Amazon's mobi library used in kindlegen and on Kindle, not baf's libmobi or KindleUnpack) it would be nice to have [also] MIT/BSD-licensed library code as an implementation example compatible with most of possible licenses for further (re)implementations (as opposed to end-user programs, where differences between GPL and MIT/BSD aren't important in this sense). For me, this case is about disseminating knowledge in source code form and not necessarily about freeing software.
Anyway, I didn't want to start flamewar about licensing and didn't mean to thrust my opinion on it. Sorry for raising this theme.
As a sidenote: I don't want to read and reimplement (L)GPL-licensed mobi-related code mostly because of moral considerations, not legal. I don't think somebody will really sue me, if I'll read your code, get knowledge about algorithms and data structures, implement them and share result under MIT license. It's just that such reimplementation not in the spirit of principles behind GPL and "free software". Also I didn't even start to write my code, so following this reason is easy and rational ATM
UPD: to be fully clear, (L)GPL of existing source in this concrete case is acceptable for me, and I don't want to persuade you to change it. I respect your personal choices.