First, my favorite company for physical game guides is Prima.
Second, the one series that a guide is awesome for is Animal Crossing!!! Keeps track of everyone's birthdays, events, furniture, bugs, ect. There is just way too much in that game to keep track of at once, and having the prima guide on hand was very helpful. (My brother bought it when we both were playing ACNL together)
Other than that I will use a guide if I well and truly stuck, have finished the game and have already spent some time tracking down 100%, or am playing really old games, in which case number one usually applies!
