RxDOS

Dissecting DOS: A Code-Level Look at the DOS Operating System
By Michael Podanoffsky; Addison-Wesley, 1994, ISBN 020162687X, has CD-ROM. Explores DOS structure, function from programmer view, overall architecture, command structure, code-level details of vital actions. [Amazon.com]
http://www.amazon.com/exec/obidos/ASIN/020162687X

RxDOS
Fast industrial strength DOS compatible RTOS, supports huge disk drives, FAT32 volumes, Windows 95/98 long filenames, coded in Assembly, used mostly in embedded systems and similar uses. [Open Source, GPL]
http://sourceforge.net/projects/rxdos/

RxDOS Downloads
Several Zip format files: readme, bootable MASM files: RxDOS sources, binaries. Another OS: RTX, fast, multithreaded for DOS in C.
http://rxdos.sourceforge.net/

Samsung Computer Division: Products: RxDOS Series
Instructions on making a bootable RxDOS floppy, and these files: disk image, Zip files with executable binaries (old, newest), GPL license. Useful for updating BIOS or MICOM.
http://www.samsungpc.com/gb/support/rxdos/main.html