Apparently there are quite a few free ebooks out there on the web. This is the third find in a few days. This free ebook is authored by James Chronister and it is available through the website of the Central Dauphin High School.
There are two editions available: the 3rd edition covers version 2.49, the 4th edition covers version 2.5/2.6. I checked the 4th edition, at first glance the book covers lots of topic in 24 chapters and 178 pages. It comes with color pictures while some video tutorials are available on the web. The file is in the usual PDF format.
The download page is a bit crowded, so you’ll have to visually scan the page to find the link to download the book. Also available on the page are links to a material library and a 3D trees library. A translation of the book in Russian (no edition specified) is also available.