It's not always easy to tell a good book from a bad one.

Literal

Good-book [and-と] bad-book [object-を] distinguish [nominalizer-のは] not-necessarily easy not.

~を見分ける ('to distinguish X, tell X apart'). 必ずしも~ない is the 'not necessarily, not always' pattern — qualifies a universal claim. 容易 is a formal な-adjective for 'easy' (contrast with 簡単).