I just wanted to comment to bump this post, because this book is so great. Such a good read, never dry or boring, and never over my head, yet so informative. I love how he takes the reader along through his learning and discovery; it's not just science, it's also kind of an adventure. Apparently he's written other books as well, which I am going to try to find and read, because I really like his style.
The above, more recent comments give the current, active link to read it online, but here it is again to make it simple; this link is active as of today
http://journeytoforever.org/farm_library/weeds/WeedsToC.html
The incredible thing is the copyright for this book is 1950, yet these ideas are still not mainstream. Pretty sad, but I am glad to be learning this information now, anyway.