I always bring a Lonely Planet guidebook with a Japanese guidebook when I make a trip.
Unlike Japanese ones which are full of neat photos, Lonely Planet books are packed with many letters with a few photos. However, LP really gives me interesting information, especially history, about the region I am visiting. Fun.
LPs in my bookshelf sometimes remind me of the past trips and let me take a photos of all LPs I brought except for books missing or given to local people.