Impossible to answer in isolation. You are looking at the question backwards. What you should be asking is "how can I solve the specific problem XYZ", and if the solution requires a lego champ AND that lego champ either needs books (generally, to reduce cooldowns or increase chances of important debuffs landing) OR benefits in other important ways (generally, on damage-dealing champs, boosting their damage output), then and only then do you use the books.

My bad, I will be more specific next time; sorry Kramaswamy.Kr >:D<