。
You mustn't re-lend a book.
Literal
Book [object-を] re-lending do-[forbidden-てはいけない].
又貸し ('re-lending, sub-lending') is a compound — 又 ('again') + 貸し (noun stem of 貸す 'to lend'). 又貸しする means to lend something you yourself borrowed. ~てはいけない is the compound prohibitive: ~て form + は (contrastive) + いけない ('won't do / is no good'). The standard 'must not / shouldn't' construction.