(My answer have some limitations, because I myself can't answer beyond that, but without going there if you read my answer you will probably get the answer of your question, not too deep though.)
Do you have [Noun Phrase (NP)]?
Now this NP here is -

Here the PP (prepositional phrase) is the complement of the head noun (suggestion). A PP often acts as a complement or postmodifier of a noun phrase.
The prepositional phrase structure is like this -

There has to be a head preposition - as to/about. Without it the phrase structure is incorrect. And the complement to the head preposition is an embedded wh question.
So from it, it's clear that you can't omit as to or about from the sentence, or it will make it ungrammatical.
Without the preposition there is no connection between the head noun and which book I should buy part. A preposition connects them together.
(My answer doesn't address why the complement to the head noun in the NP tree has to be a Prepositional Phrase. The simple answer is I don't know. I couldn't find the reason beyond what I said here in any grammar book I checked, though I haven't checked many books. I think this will be dealt with not in grammar books, but in good linguistic books, and I don't have any. Nor do I have any knowledge in linguistic theory. Someone else might answer it better.)