One of the best books for UNIX internals is:
The Design and Implementation of the FreeBSD Operating System
Marshall Kirk McKusick and George V. Neville-Neil
ISBN: 0-201-70245-2
Other good internals books are:
Solaris Internals: Solaris 10 and OpenSolaris Kernel Architecture
Richard McDougall and Jim Mauro
ISBN: 0131482092
Other good internals books are:
An older book, but covers several operating systems:
UNIX Internals The New Frontiers
Uresh Vahalia
ISBN: 0-13-101908-2
The book for UNIX programming:
Advanced Programming in the UNIX Environment
W. Richard Stevens
ISBN: 0-201-56317-7
This is one of the best books on C programming. It discusses many topics which are not documented or explained very well. A must buy for any C programmer.
Expert C Programming Deep C Secrets
Peter Van Der Linden
ISBN: 0-13-177429-8
A great book on designing and creating reusable software in C.
C Interfaces and Implementations: Techniques for Creating Reusable Software
David R. Hanson
ISBN: 0-201-49841-3
The Practice of Programming
Brian W. Kerninghan and Rob Pike
ISBN: 0-201-61586-X
Algorithms Sequential & Parallel: A Unified Approach
Russ Miller and Laurence Boxer
ISBN: 1-58450-412-9
UNIX Network Programming: Networking APIs: Sockets and XTI
W. Richard Stevens
ISBN: 0-13-490012-X
Great book on performance techniques and analysis:
Performance by Design
Daniel A. Menasce, Virgilio A.F. Almeida, Lawrence W. Dowdy
ISBN: 0-13-090673-5