Honestly, we found so much information including maps and directions on line or from publications available at no charge by ordering on line that we did not need any guidebooks. And, once on each island, the airport stands and other places carry "101 Things to Do on _____(whatever island)" which is also informative.

However,if you do want a guide type book, any good bookstore will probably carry something like Fodor's Hawaii or similar...

....But, I would opt to buy a really nice Cocktail Table Book which will make a great gift and will probably include photos or data on places they go...and it will be a lovely remembrance and addition to their home.

Good Luck, Lucille in Ct