The Single Book Answer,
Rezension bezieht sich auf: Advanced Programming in the UNIX Environment (Addison-Wesley Professional Computing) (Gebundene Ausgabe)
All the other praise is well deserved. This book sets an incredibly high precedent as to the quality of technical literature. GET THIS BOOK. The UNIX API has never been presented better.
My only single complaint is that the text is written as a programmer would have written it: that is, much passive voice. Just make sure you have some coffee and a lot of time. It took me a little time to get through it (a 3 day cruise in the bahamas, actually), but I am to this day thankful that I did.