If you ask us, and we do get asked a lot, the best book to learn about farming and gardening (or to learn a few new tricks if you're a life long farmer or gardener) is to go to the source: Jethro Tull's Horse Hoeing Husbandry, the first, and still the best, book on modern scientific agriculture.  It's available on Kindle for just under $2 at

http://www.amazon.com/Hoeing-Husbandry-Jethro-Brachfeld-ebook/dp/B0041OSBPW

The book teaches you how to farm and garden better - and the science behind why.  Covering everything from soil science to harvesting and marketing, from training draft animals to training better roosters, the most basic algae to the most complex agroecology, the history of agriculture gains new relevance when updated with modern science.