Diaspora by Greg Egan is my very favourite book.

It explores in depth identity, consciousness and transhumanism, as well as need for exploration, fresh ideas on alien species and human relationships. [Link](http://www.bookdepository.com/Diaspor-Greg-Egan/9781597805421?ref=bd_recs_1) I really recommend the book to anyone who likes to think and be challenged. At the same time it is actually fun sci-fi adventure book, that keeps the ideas from being presented in tedious way.