。
Reading books is really interesting.
Literal
Book [object-を] read [nominalizer-の]-[topic-は] very interesting.
The pattern [verb plain form]+の+は...だ is the standard verb-phrase nominalization: verb + の (nominalizer) turns the phrase into a noun ('the act of reading books'), then は marks it as the topic. Equivalent to English gerunds. The alternative nominalizer is こと; both work for most verb phrases, but の feels more immediate/concrete while こと is more abstract.