I can't think of another photobook that has had as much critical, scholarly, and aesthetic attention as Frank's The Americans. Reprinted a number of times (most successfully in the most recent Steidl edition), this is the ultimate reference on The Americans, including not only all of the original images from that book, but a wealth of supplementary images, critical essays, and so forth. The best thing about the HC volume (ommitted from the softcover!) are the contact sheets on which the images from the book appear. This gives a fascinating glimpse into Frank's editing process, and you realise from this just how different the The Americans could have been. So for a few quid don't buy the softcover! I'd recommend getting the new Steidl edition (even if you're lucky enough to own a first edition!) so that you can have both Looking In and The Americans open at the same time.
The critical essays are insightful, if sometimes a bit wordy. My only disappointment with this book was that it's so heavy it's easily damaged in shipping, and mine arrived with one corner fairly badly bumped, but I didn't notice until after I'd taken off the plastic wrap.