Sentence:  In short , the book , based largely on lectures delivered at Harvard University , <WSD>is</WSD> both reliable and readable ; the author possesses an uncommonly fine English style , and he is dealing with subjects of vast importance that are highly topical for our time . 
Meaning of the word marked with <WSD> tags:  have the quality of being; (copula, used with an adjective or a predicate noun)