I have purchased Thom's e-books since the D-200 came to market. With his e-book, you will receive a CD in the mail (no downloads), which you can load onto your computer or have printed at your local Kinko's. Personally, I like loading the pdf file onto my field laptop computer. The pdf is searchable, which makes it truly a quick-reference for the field, where you need information fast, in most cases. Thom also includes a printed quick-reference guide that is just slightly larger than pocket-size.
Lastly, Thom's blog on the front page of his website is a good place to learn what is going on in the digital imaging world at any given time. Check it out!