。
Reading books is interesting.
Literal
Book [object-を] read [nominalizer-こと]-[topic-は] interesting.
Verb-phrase nominalization with こと instead of の. Both nominalize verb phrases: こと is more abstract/conceptual while の is more concrete/immediate. Some constructions require one or the other (~ことができる takes こと; ~のを見る often takes の). Here either works, but こと has a slightly more general, philosophical flavor — 'reading as a concept.'