the most interesting book (who, which, that) I've ever read.

[keannu]

Can only "that" be the answer here for the superlative adjective? Doesn't "which" also work here?

37)This is the most interesting book (who, which, that) I've ever read.

 

[bhaisahab]

Can only "that" be the answer here for the superlative adjective? Doesn't "which" also work here?

37)This is the most interesting book (who, which, that) I've ever read.

It would be more natural without any of them: "This is the most interesting book I've ever read."

 

[Rover_KE]

I agree with bhai. If you must choose, use 'that'.