Ok, so if there were a single book you were to buy on cycads, this would be my recommendation. It has EVERY species in it (at least known at the time of printing... are more now) AND it is likely the author has had at least some personal experience with each species, or at least someone he knows well has. There is probably no one in the world who knows more about cycads than Loran Whitelock. He is a self trained biologist and probably one the most knowledgable individuals concerning Cycad taxonomy and cultivation, as well as cycad biology. He knows his plants, and his book shows it. There is an excellent and thorough discussion of every single species AND a photograph of nearly all of them, too, most in the wild (so you get to see what environment these plants grow in). The photos are from cycad experts all over the world, and are alone worth the price of the book.
The book has some nice pen and ink drawings, if I say so myself (mine), and excellent appendeces with landscaping tips and groupings of plants based on color, shape, size, climate etc.. It is excellent reading, not too technical, and a very nice quality binding.
My only complaint about this book is the photos are all in one section (I know it is cheaper to do it this way, but it's too bad, too), and not spread out throughout the discussion.
However, I highly recommend this book to anyone interested in cycads, and consider it THE book.