Probably that should be is because the range is available. That of books part is not the head noun of the noun phrase, and the plural part is only the prepositional object. It is the head noun which is the subject whose number governs that of the verb, and that is range which is normally singular.
You’ll hear people say it the other way often enough, though, and there are cases where it actually does go the other way. You have to think about it a bit sometimes.
It may be that the speaker is thinking of range of the way they think of lot of, which would mean that range of would now be a pre-modifer to the (logical) head noun books, leading in that case to a plural, just like it does with a lot of people are as opposed to a lot of life is. But that might be hard to argue.
I’m not completely certain where superlative adjectives enter into this picture. I’m afraid any simplistic rule about them may lead you into confusion and error. Consider:
- The smartest children are the first to leave.
- The smartest are the first to leave.
- The smartest child is the first to leave.
- The smartest is the first to leave.
All are fine. The verb tells you whether the subject is being construed in the singular or plural. This is the speaker’s choice, depending on intent.