I buy several guide books for somewhere I haven't been before, the number depending upon how long we will be there. (Had 5, 3 purchased, 2 gifts, for the 2 1/2 weeks we spent in Spain last year. Took 2 with us. On the other hand, going to San Fran. and Monterey next month, and won't have a book--have been there so many times.)
However, I have a whole shelf of them. Can't bear to part with them though. I greatly enjoy looking at them every once in a while, when thinking of where to go on the next trip, or whether to go back to a place we have already been. I also loan them out to many of our friends traveling to places we have already been, or just consult them to offer tips to friends who ask us about traveling to a specific location.
Sometimes I buy them before I even have a trip planned to the location, as I enjoy reading about places I am thinking of going. (Have a couple on Portugal on my shelf as we speak, and it's on my list.)
Djlawman