Check out http://wpebooks.com/2010/09/how-to-enable-multisite-in-wordpress/#comment-478
There are two ebooks on WP multisite that I have bought and can recommend
The first book is by Andrea Rennick and is a short guide to enabling WP multisite. Its clear and well written, easy to follow and explains the basics of WP multisite. I knew most of it but it was a good refresher to check my understanding of the key concepts.
The second book is one that has been co-authored by Mika Epstein and Andrea Rennick and is a more in depth look at WP multiple networks and multisite… I'm only part way through but I like the easy to follow style of the book. I'm learning heaps about WP multiple networks and multisite and I hope I can learn enough to be confident to manage my WP multiple networks / multisite installation. You can find more info about the second book here http://halfelf.org/ebooks/wordpress-multisite-101/
Both come in Kindle format for use on a Kindle or, as I do, on an iPad with the Kindle app installed. There are also PDF and ePub formats.
Good work Andrea and Mika I love your work :)
p.s. I came across Mika on some of the Wordpress.org support forums… a very knowledgeable and helpful person :)
See http://wordpress.org/support/topic/warning-upgrade-to-37-killed-my-multisite?replies=12#post-4832058 for an example