If you're serious about programming your Pi then you'll need some books. The ones I recommend are shown here.
Ubuntu Linux Toolbox: 1000+ Commands for Ubuntu and Debian Power Users - ISBN 0470082933
Ubuntu Linux Toolbox: 1000+ Commands for Ubuntu and Debian Power Users by Christopher Negus and Francois Caen (ISBN 0470082933) provides an excellent concise summary of Linux commands. Available from Amazon.
Raspberry Pi User Guide - ISBN 111846446X
Raspberry Pi User Guide by Eben Upton and Gareth Halfacree (ISBN 111846446X) provides the essential guide to getting started with your Pi. It explains all about the Pi, setting it up, Python programming, GPIO, screen sizes, etc. Available from Amazon.
Raspberry Pi Manual: A practical guide to the revolutionary small computer - ISBN 0857332953
There is now a Haynes manual on the Raspberry Pi. It's called Raspberry Pi Manual: A practical guide to the revolutionary small computer (Owners Workshop Manual) by Gray Girling (ISBN 0857332953) and provides the essential guide to interfacing your Pi to other devices - it's more of a cookbook than the User Guide above. Available from Amazon.
Threads Primer - ISBN 0134436989
The first is Threads Primer and is one of a series of technical programming books published by SunSoft Press and Prentice Hall for Sun Microsystems.
Providing an overview of the Solaris and POSIX multithreading architectures, this book explains threads at a level that is completely accessible to programmers and system architects with no previous knowledge of threads. Covers the business and technical benefits of threaded programs, along with discussions of third party software that is threaded, pointing out the benefits.
Programming with Threads - ISBN 0131723898
Programming with Threads is of a more advanced nature and deals with typical problems, applications and has plenty of examples.
This practical guide to developing multithreaded programs on UNIX, concentrates on when and how to apply threads, what problems to expect and how to deal with them. It covers advanced interfaces appropriate to performance-critical situations, explores the use of threads, and the construction of correct MT programs and libraries. The text also includes a variety of multithreading examples.
Deep C Secrets - ISBN 0131774298
If you're going to be programming threads in C, then Deep C Secrets is one book that I can recommend for a glimpse of Peter van der Linden's real-life experiences with C. It covers programming techniques, real-life examples of things that went wrong, lots of code snippets and examples and presents them in a conversational and easily readable style that'll make you wonder why they don't write books like this today.