Learn English – Are “skill set” and “skill sets” both acceptable

There may be a transatlantic difference here. The Corpus of Contemporary American English shows that ‘around’ is used almost nine times more than ‘round’. The British National Corpus shows ‘around’ to be used less than one and a half times as much as ‘round’. The OED suggests that ‘around’ might have originally been a British English usage, and that it is now returning under influence from American English. Anyway, the OED shows them to be synonymous.

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.