There are untold numbers of USDA Bulletins out there, and they’re a wealth of free information, but I have been stymied in my search for a comprehensive online collection. I’ve found lists of all the bulletins ever published, but can’t find a single source for purchasing or downloading them. However, I recently stumbled upon one place to download quite a few of them:
the USDA Home and Garden Bulletin wing of the The Greenstone Digital Library. [Note from the future: the bulletins are now found here on the USDA website. This link will open search results.] It’s not comprehensive, but it has some great titles dating back to the fifties.