Operating System Design
Comer, Douglas E., Operating System Design: The Xinu Approach, Prentice-Hall, 1984.
This is the essential text to understanding the Xinu operating system. After pouring over this, you may want to write a device driver for ATmega ADCs.
Try looking for a used copy - you can probably find one in perfect condition for about $6.
Operating System Design - Volume II
Operating System Design: Internetworking with Xinu, Volume II, Prentice-Hall, 1987.
Ethernet, packets, UDP datagrams, and a complete file server to run on your host computer. Is it all outdated? I don't think so. I've replaced the RARP client with a DHCP client. You don't need a time server running on your local network anymore, just address a NIST server, but there's a simple time server included in the avr distribution just in case. Avr-Xinu in this distribution is derived from XINU-68K.TAR.Z dated 12/29/1995.
It's not often that you find software so well documented.
Internetworking with TCP/IP - Volume II
Internetworking with TCP/IP: Design, Implementation, and Internals, Volume II, Prentice-Hall, 1991.
This reference includes source code for most protocols including ARP, TCP, IP, RIP, and SNMP. The Xinu operating system is upgraded to Version 8. This version has not been ported to AVR microprocessors mainly because of its need for considerably more RAM than Version 7.
here. You need WinAVR for Windows.
Comer, D., Operating System Design - The Xinu Approach, Linksys Version, CRC Press, 2011, ISBN 978-1-4398-8109-5.