This is a great site (a virtual library) with links to the entire texts of some classic books in organic and natural gardening, many of which are now out-of-print and sadly unavailable:
Click here to access Holistic Agriculture LibraryAmong the texts you can read are classics like Cato on Farming, many of Sir Albert Howard's papers, and Charles Darwin's The Formation of Vegetable Mould through the action of worms with observations of their habits. London, 1881.
We found it looking for more information on Masanobu Fukuoka. He is one of the pioneers of no-till grain cultivation. His system is referred to as "natural farming", Fukuoka Farming, or the Fukuoka Method.
We were reading his book "The One-Straw Revolution," which you can now find in some bookstores as it was brought back in print last year. His method of farming transcends organic and follows natural ways from broadcasting seeds and always keeping the soil covered.
Enjoy!