This is a very good question and many people wonder about this structure.
The pronoun "welches" is singular and it's something special (unsystematic) in German. It can also refer to nouns in the plural. But this is only possible, if it's alone (far from the noun after which one asked) and the verb "sein" is associated with a noun in the nominative.
We distinguish between the following cases:
1) It is asked for a noun in the singular:
Example: Welches Buch liest du?
Explanation: The "book", the noun which is asked for, is directly behind the question pronoun
and is singular.
2) It is asked for a noun in the plural:
Example: Welche Bücher liest du?
Explanation: "Books" is plural and stands directly behind the question pronoun.
Example : Welches sind die Bücher, die du liest?
Explanation: The noun "books" is linked to the question pronoun via the verb "sein" and is
not directly behind it. Only in such cases we use "welches". The sentence ,
you asked for, follows this rule and therefor it's correct to use "welches.