Following books helped us a lot (I should say that they are solid knowledge foundation):

• Programming:
  • "The C Programming Language"
    Brian W. Kernighan, Dennis M. Ritche
    ISBN: 83-204-2719-3
  • "Beginning Linux Programming"
    Neil Matthew, Richard Stones
    ISBN: 83-7243-020-9
  • "Advanced Linux Programming"
    Mark Mitchell, Jeffrey Oldham, Alex Samuel
    ISBN: 83-7243-217-1
  • "Professional Linux Programming"
    Neil Matthew, Richard Stones
    ISBN: 83-7197-495-7
  • "Linux Kernel Development"
    Robert Love
    ISBN: 83-7361-439-7
  
  
• Programming techniques and theory:
  • "Introduction to Algorithms"
    Thomas H. Cormen, Charles E. Leiserson, Ronald L. Rivest, Clifford Stein
    ISBN: 83-204-2879-3
  • "Data Structures and Algorithms"
    Alfred V. Aho, John E. Hopcroft, Jeffrey D. Ullman
    ISBN: 83-7361-177-0
  • "The Design and Analysis of Computer Algorithms"
    Alfred V. Aho, John E. Hopcroft, Jeffrey D. Ullman
    ISBN: 83-7197-770-0
  • "Programming Pearls"
    John Bentley
    ISBN: 83-204-2672-8
  • "Języki Programowania"
    ToMasz Wierzbicki
    Symbol Uniwersalnej Klasyfikacji Dziesiętnej: 004.43
    2000 Mathematics Subject Classification: 68N15
  • "Operating System Concepts"
    Abraham Silberschatz, Peter Baer Galvin, Greg Gagne
    ISBN: 83-204-2961-7
  • "Principles of Concurrent and Distributed Programming"
    M. Ben-Ari
    ISBN: 83-204-1996-4
  • "Hacker's Delight"
    Henry S. Warren, Jr.
    ISBN: 83-7361-220-3
  
  
• Software engineering:
  • "The Art of UNIX Programming"
    Eric S. Raymond
    ISBN: 83-7361-419-2
  • "The Pragmatic Programmer"
    Andrew Hunt, David Thomas
    ISBN: 83-204-2672-3
  • "Writing solid code"
    Steve Maguire
    ISBN: 83-7197-429-9
  • "The Mythical Man-Month"
    Frederick P. Brooks, Jr.
    ISBN: 83-204-2467-4
  • "The Practice of Programming"
    Brian W. Kernighan, Rob Pike
    ISBN: 83-204-2732-0
  • "Extreme Programming. Pocket Guide"
    Ward Cunningham
    ISBN: 83-7361-343-9
  • "Inżynieria Oprogramowania"
    Andrzej Jaszkiewicz
    ISBN: 83-7197-007-2
  
  
• Networking:
  • "TCP/IP Network Administration"
    Craig Hunt
    ISBN: 83-7243-305-4
  • "TCP/IP Bible"
    Rob Scrimger, Paul LaSalle, Mridula Parihar, Meeta Gupta, Clay Leitzke
    ISBN: 83-7197-668-2
  • "Teach yourself TCP/IP in 14 days"
    Timothy Parker
    ISBN: 83-86718-55-2
  • "Introduction to the Internet Protocols"
    Charles Hedrick
    
  • "Connected: An Internet Encyclopedia"
    Brent Baccala
    
  • "Internet Protocols. Description and Packet Format"
    alex@netfor2.com
    
  • "Internet Tutorials"
    InetDaemon Enterprises
    
  • "Programming with pcap"
    Tim Carstens
    
  
  
• Miscellaneous packages and libraries documentation:
  • "GNU Autoconf manual"
    Free Software Foundation, Inc.
  • "NCURSES Programming HOWTO"
    Pradeep Padala
  • "Writing Programs with NCURSES"
    Eric S. Raymond, Zeyd M. Ben-Halim, Thomas Dickey