I agree with Jeff, it depends on how much you know already. If you've already built 5 or 6 different hammocks and own 3 or 4 store boughts, then maybe you don't need it. For me, it was Ed's book that actually got me camping in a hammock. I'd look at pictures of the things and think there's no way I could sleep in that banana position! and no way I was going to spend a lot of $$ to check it out. I saw and bought Ed's book and made a test hammock for 5$ from material from wallyworld. It took maybe a half hour (didn't even hem that first one). Boy was I surprised when I got in it! Anyway, Ed's book takes you thru step by step, in a logical order, helps you figure dimensions etc. Plus there a lot of tips about hammock camping, where to hang etc. I felt like he went out of his way to not show any bias against other hammock manufacturers. Nothing against Hennesey, but you can't buy a book off Tom Hennessey telling you how to make one yourself!
Beware, Ed's book will cost you more than the intitial purchase price. Because of it, I now have 5 hammocks, multiple under quilts, pads, tarps, etc., with an order in for the netless claytor. I figure Ed's book has cost me over a thousand dollars at least, but the enjoyment and comfort I received....priceless (sorry couldn't resist the cliche)