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/search
Many 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'.
... and there is also some interesting stuff if you search on 'agriculture', including:
'The $100 Prize Essay on the Cultivation of the Potato; and How to Cook the Potato, by D. H. Compton and Pierre Blot'
here:
http://www.gutenberg.org/files/25905/25905-h/25905-h.htm
Tip for downloading the books - go for HTML or plain text - I myself have no idea what 'plucker' format is, so can't advise you about that one. Once you open the page in html or plain text, you should be able to save the document using the 'save as' command.
Goodness there,s alot to explore on here ! I,ve saved it to browse though another time. Thanks for that ! Debs :)