Title: PaulOS
Subtitle: Part I An 8051 Real-Time Operating System
Description: This text book is intended for under-graduate Engineering and/or IT students studying micro-controllers and embedded systems. It covers mainly the 8051 family of micro-controllers starting with the basic architecture and then moves on to assembly language and Real-Time Operating System (RTOS) where three main types of self-developed RTOSs are discussed (with full source code listing). 

You can download the book for free via the link below.