You should read the book and see what he means/meant by......
Question: Do we use present or past tense for a book? And does it matter if it's fictional books (Shakespeare) or fact (political/religion)
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Either "means" or "meant" is acceptable.
If you were being really fussy, you might say
You should read Keynes's book "The Theory of Money" and see what it means
The intended meaning of a book's text does not change over time, and so its intended meaning is now what it ever was. Or you might say
"You should read Keynes's book "The Theory of Money" and see what he meant
Obviously Keynes does not currently intend any meaning today: he is dead. Moreover, it is conceivable (indeed highly likely) that Keynes changed his mind after writing that book and did not even intend that meaning at times while he was alive but had finished that book.
But, as I said, this is being very careful in what you write. No one should fault you for using "it means" or "it meant."