Embedded: eCos (Embedded Configurable O.S.)

eCos is a free open source real-time operating system intended for embedded applications. 
You can find a free eBook for eCos from the below link:

EMBEDDED SOFTWARE DEVELOPMENT WITH ECOS™
  => http://ptgmedia.pearsoncmg.com/imprint_downloads/informit/perens/0130354732.pdf

Wikipedia page of it: http://en.wikipedia.org/wiki/ECos

List of real-time O.S.: http://en.wikipedia.org/wiki/List_of_real-time_operating_systems

Embedded Linux or eCos?: http://stackoverflow.com/questions/2672502/embedded-linux-or-ecos