Hún spyr hvort „stelpan“ sé frumlagið og hvort „bókina“ sé andlagið.

Here hvort introduces an indirect yes/no question, so it means whether.

So:

• Hún spyr hvort ... = She asks whether ...

In English, if is sometimes possible in similar sentences, but whether is the closer match here.

Sé is the present subjunctive form of vera (to be).

It is commonly used in indirect questions and similar subordinate clauses after verbs like spyrja (to ask), especially in more careful or standard written Icelandic. So:

• hvort ... sé = whether ... is

If you know er as the normal present form of to be, that is correct for many main clauses, but here Icelandic often prefers the subjunctive sé.

It is repeated because the sentence contains two parallel indirect questions:

• whether stelpan is the subject
• and whether bókina is the object

Repeating hvort makes the structure clear and balanced. English often does the same:

• She asks whether A is the subject and whether B is the object.

In some contexts, repetition might be omitted, but here it is natural and clear.