。
Books are like friends.
Literal
Books [topic-は] friend like-[attributive-な] thing [is-である].
みたいな ('like, similar to,' the attributive form of みたい) connects 友人 to the generic noun もの. The pattern XみたいなものだX is a common way to assert a metaphorical or approximate identity — 'X is like a Y, basically.' Final である is the formal written copula, giving the sentence an essay-like register.