Swan categorizes the word according to its referent, Thomson & Martinet categorize it according to its syntactic role. Neither categorization tells you much that's useful, and both reveal the inadequacy of the traditional 'part-of-speech' approach.
In its core use, designating the location or goal of an entity or action, where is neither a pronoun nor a [pro]adverb but a pro-PP—that is, it 'stands for' a preposition phrase constructed with
a) an unspecified-but-inferrable preposition—in your first sentence it's in, in your second it's at—and
b) an unspecified-but-inferrable relative which
- a village where there are no shops = a village in which there are no shops
- the hotel where they were staying = The hotel at which they were staying
In these cases Swan's categorization is clearly inadequate, and T&M's categorization may or may not be accurate: locative PPs do not always act as 'adverbs' modifying clauses. In 2., for instance, the PP represented by where should probably be regarded as a locative complement, not an adverbial.
And there are also uses of where to designate the origin of an entity or event; here where has to be supplemented by an express preposition such as from:
The town where he comes from . . .
In informal registers this supplementary preposition is often extended to 'core' locative and goal uses:
Come on, come on, let me tell you where it's at
In such cases where may be described as a relative 'pronoun' with regard to its referent, and in most cases it will act as a locative complement rather than as an adverb.
