this is an article usage, mainly.(the)
She is the best and the wisest girl in the class.
From the ground rule of article usage, 'the'+adjective is used if the adjective in a sentence is in superlative form. For example:
- The best
- The baddest
- The worst
- The richest
- The wisest
Your rule book is right, stick into it. The best and the wisest in the class, means amongst other students in the class she is the best(superlative) and the wisest(superlative). Alrighty?
On the other hand, if you are referring to the 'best and wisest' it is another topic, because there are common 'set phrases' such as 'the best and cheap cellphone' as you can see 'cheap' is not in superlative form here.
Further, if you see some sentences with 'the best and (adjective)... -they are mostly set phrases or idioms, so, they are exception to the rule of article usage