One online "book" (MSWord document) is the Tics RTOS Book here:

http://www.concentric.net/~tics/

Along with some basic articles:

http://www.concentric.net/~tics/tutorials.html

You have not really specified your level of interest. If you a interested in designing an RTOS (i.e., you are going to write one), then Labrosse's "MicroC/OS-II" is decent. Of course non-RTOS classics like Tanenbaum's "Operating System Design and Implementation" apply in this case too.

On the other hand, if it's using an RTOS that you desire to understand, some older books that might be worth reading (in order of preference) are:

• "Real-Time System Design" by Levi & Agrawala, ISBN: 0-07-037491-0 This book is more rigorous and academic in its presentation.


• "Real-Time Software Techniques" by Heath, ISBN: 0-442-00305-6


• "An Implementation Guide to Real-Time Programming" by Ripps ISBN: 0-13-451873-X

A newer introductory book is "Real-Time Concepts for Embedded Systems" by Li, ISBN: 1-57820-124-1.

Most RTOS's are designed for event-triggered systems. However, Michael J. Pont presents time-triggered systems in his book "Patterns for Time-Triggered Embedded Systems" ISBN: 0-201-33138-1:

http://www.le.ac.uk/engineering/mjp9/pttes.html

Here is a fairly comprehensive URL list of commercial RTOS's and following the links might give you some more resources:

http://www.realtime-info.be/encyc/buy...r1048.html

E. Douglas Jensen's http://www.real-time.org has a Real-Time Resources link where you'll find more book references and has a Real-Time Outline link discussing fundamental concepts.

Edit: Please try to review the aforementioned books at a library before buying them.