Of the books I recall having in my personal library (I'm not near it at the moment), I can find none online. However, here's a URL for you to check out:

http://www.dhruvaraj.com/olbooks/O...mbRtos.htm

Some of the links therein are to books; some are to product manuals. Of the books, I have not read any and I have to say that I am dubious as to whether they, due to copyrights, are eligible to be online. Of the product manuals, I have read the VxWorks manuals as needed to perform my work. The manuals are just that -- manuals -- and are not necessarily suitable for learning RTOS concepts.

When I have access to my library again, I will make a point of looking for online copies of books in my (RT)OS section. If I find anything, I'll post again (this might be days from now).

Pumpkin Inc.'s Salvo User Manual (and the Lite version of Salvo itself) is free and has an "RTOS Fundamentals" chapter, which I have read. It's a decent introduction, but does not cover the subject in depth. However, since Salvo Lite is free, you can experiment with the multitasking concepts presented, but you should be aware that a cooperative multitasker like Salvo is more of a "communicating concurrent state machines" abstraction (having both pros and cons) and imposes more coding restrictions than a preemptive multitasker would. Here's a URL where you can download the Salvo User Manual:

http://www.pumpkininc.com/content/user_manual.htm