If the guy already knows how to play a guitar...I don't think a guitar book would really be suitable. With any how-to book they're usually geared more towards beginners.

A cake is always nice. I hate cake but it's the gesture that they took the time to make it that's sweet. What other things does he like? Find some place where you two can go at night to see something nice. I loved doing that kind of stuff. Like going out to high places to overlook the city or seeing the sunset with ice cream or coffee, us guys like romantic stuff as well every now and then.

But a book is a good idea, and gift receipts are good things as well just incase.