。
Books and friends should be few in number and good in quality.
Literal
Books [conjunctive-と] friends [topic-は] number [subject-が] few-[conjunctive-く] quality [subject-が] good-[must-なければいけない].
A proverbial-sounding statement chaining two parallel predicates about a compound topic. 少なく is the adverbial/連用 form of 少ない ('few'), functioning here as a conjunctive (equivalent to ~て, linking adjective clauses in literary style). 良くなければいけない applies the 'must' construction to 良い ('good'), using the い-adjective's く-stem. Both predicates describe the compound topic (books and friends).