"The maximum number": plural or singular?
As explained in ‘The Cambridge Guide to English Usage, when number of is preceded by the, it is the head (the most important word) of the phrase ‘the number of . . .’ It is singular, and so requires a singular verb.
The case is different when number of is preceded by a. Then, it is a pre-modifying element, leaving the number of the verb to determined by the number of the following head word (wordbooks in your example), which will invariably be plural.