They had almost reached the door when a voice spoke from the chair nearest them, "I can't believe you're going to do this.”
I guess nearest is at the place of preposition. Can a preposition have the form of superlative?
Near is a bit of an unusual 'frozen' word. It was originally the comparative form of nigh (from OE nior). The terms nearer and nearest came later as speakers reinterpreted near as a positive form.
In addition near is rather vague with respect to its word class. The OED Online at near, adv.2 (and prep.2) notes the "difficulty of distinguishing the adjectival, adverbial, and prepositional or quasi-prepositional uses" of the word.
Regarding the comparative and superlative "prepositional" uses, the OED (ibid.) says:
...(emphasis mine) which is exactly the case here. In short, it looks like a preposition, but it's a sneaky adverb.
Also, at least in etymological origin, the to in "near/-er/-est to them" was actually a later addition to the idiom from Middle English, so it's not well-motivated to regard "near them" as a to-deletion as opposed to retention of an earlier form.
It is a clever example, but most will consider this to be a shortened form of nearest to them. Adjectives can take prepositional phrase complements, and of course are the only word class which takes superlative marking.
This is a nice example of how linguistic data is frequently "corrected" by linguistic theory.
This is a good example of how useless traditional parts of speech are.
As usual, many markers have been deleted from a clause, producing an opaque (but much shorter) result. Below, the deleted markers are in boldface:
As you can see, nearest is not a preposition, but a simple predicate adjective which happens to be inflected with the superlative -est. The repeated NP the chair is deleted in context, leaving only nearest to them, which could be perfectly ordinary in a relative clause
And then Whiz-deletion gets rid of which was, thus destroying the evidence of the relative clause, and producing what looks like it might be a very strange prepositional phrase indeed. But isn't.
Prepositions are thrown into the adverb category. Some adverbs have comparative and superlative forms, yes, and some, eg to, by, for. from, don't. "...from the chair nearest (to) them" is grammatical English.
But "near" in the phrase a voice spoke from the chair nearest them functions as a locative adverb as well as a preposition. "From" is the first preposition in that prepositional phrase, and "nearest (to)" is the second (but the to has been elided, and whether or not it's there is a matter of style and idiom).
Google Ngrams shows that both phrases are used, but that nearest you is at least 4 times more frequent than nearest to you.