Monograficos Dolmen #1: Adam Hughes
Published in Spain back in 2004, this is possibly one of the rarest Adam Hughes sketchbooks out there. Clocking at 98 pages, the (possibly unlicensed) book features every single sketch the authors were able to find on the internet, a biography, a few interview excerpts, some process info, a checklist and a nice compilation of Hughes covers.
As you can see at the bottom of the cover, it proudly announces that you will find more than 500 images inside the book, and they are not lying. There are images all over the place, every inch of the book is covered in images wherever there is no text, and isn't that what we are looking for?
Though mostly black and white (with a nice color section in the middle), the book alternates biographical information with themed galleries (The Women of Adam Hughes, Wonder Woman, Vampirella, …).
The last copy of this I have seen go for $40+ on Spanish eBay, so keep an eye out, you might find one cheap.
It is a really interesting read (if you can read Spanish), making up one of the few semi-serious books out there about Hughes and his work, starting at the very beginning (including some very early sketches) and taking us to 2004. Some of the art here might never get printed anywhere else, so there's that too.
It's also a nice reminder of Adam Hughes' work at a time when he was actually putting books out, and they were great. Hopefully those times will be back soon.