Put 'gardening' in the subject field here, and you get a list of books that can be downloaded as text documents, free:
http://www.gutenberg.org/catalog/world/searchMany of them are quite old books now out of copyright, but some seem to have been contributed copyright-free, such as Steve Solomon's 'Organic Gardener's Composting' and 'Gardening without Irrigation: or without much, anyway', which are more recent.
Plenty of the other stuff looks useful - e.g. Sutton & Sons 'The Culture of Vegetables and Flowers From Seeds and Roots', and F.F. Rockwell's 'Home Vegetable Gardening — a Complete and Practical Guide to the Planting and Care of All Vegetables, Fruits and Berries Worth Growing for Home Use'.
