If you’re comfortable traveling with your iPad (and why not?), these Lonely Planet guides look pretty amazing. Regular books seem like they’d be a pain on an iPad, but picture books for kids and interactive, picture- and link-rich content like travel guides are made for it. You can search, bookmark, leave notes, link out to maps and related websites. Wish I’d had all that with my Japan guide, which was like 700 pages long.
Each guide costs $15 — not cheap, but reasonable. They have Italy, Ireland, the UK, Spain, and France, with, I’m sure, many more planned.