1. FreeBSD Handbook
• This book is the official FreeBSD handbook. A complete and perfectly reliable installation and maintance guide for FreeBSD. Despite being an "official" book it highly readable.
• Format: portable document (pdf).
  
2. Xlib - C Language X Interface X Consortium Standard X Version 11, Release 6.4
• This book is the offical manual for X Version 11, Release 6.4.
• Format: postscript.
  
3. Linux 101 Examination Modular Training Notes
• The nice people at Leading Edge Business Solutions offer many training courses on Linux and keep the teaching material under GPL. These particular set of notes serve as an excellent reference material for the seasoned Linux User while provinding a newbee with an exhaustive information on Linux.
• Format: portable document (pdf).
4. Linux 102 Examination Modular Training Notes
• Again from the nice people at Leading Edge Business Solutions. This manual can serve as a very good reference for advanced Linux Administration and is also a good starting point in this field.
• Format: portable document (pdf).