A good book is the best of friends — the same today, tomorrow, and forever.

Literal

Good-books [topic-は] the-best [of-の] friends [copula-であり], today [also] forever-after [also] the-same [copula-である].

A literary proverb-like aphorism. である and であり are the formal copula and its conjunctive (chaining) form. 最良 ('the best') is a formal superlative. 同じ ('the same') closes the comparison.