Learn Swedish: Hon ska slå upp bokstaven i boken. - She is going to look up the letter in the book.

slå belongs to a specific subset of Group 4 verbs with the vowel pattern å - o - a. The principal parts are slå (infinitive), slog (past tense), and slagit (supine). You will see this same å - o - a vowel shift in other strong verbs ending in a vowel, such as ta (ta - tog - tagit) and dra (dra - drog - dragit).

Does the phrase slå upp follow a specific stress pattern when spoken?

Yes. Because slå upp functions as a phrasal verb (partikelverb) meaning to look something up or to open a book, the spoken emphasis is placed heavily on the particle upp, while the verb slå is left unstressed.

Why does the sentence use bokstaven for the letter instead of brevet?

In Swedish, en bokstav refers exclusively to a letter of the alphabet, like A, B, or C. If the sentence meant a written message sent in the mail, it would use ett brev, making the definite form brevet.

I see bokstaven is the singular definite form here. Does this noun have a predictable plural?