Picked this book up at Powell's recently and was pleasantly surprised with its 'Making of Uru' section. Round 100 pages devoted to the story of Cyan Worlds developing Uru and the time in between Riven and Uru (Myst 3 and 4 weren't done by Cyan). It's got some great art through out the book. I've not scanned the guide portion of the book but I did get the art from it. Please note that this was published before End of Ages was released, some of the unused content in Uru was used in it.
Uru - Complete Chronicles - The Official Guide
Also, for those that haven't seen it:
Myst Online - Uru Live Again is now free and run by Cyan Worlds. It's awesome, play it!