Learn Portuguese (Portugal): Peço o livro ao bibliotecário.

You need to replace o livro (the direct object) with o, and ao bibliotecário (the indirect object) with lhe. When you combine lhe and o, they contract into lho. Attached to the verb, the sentence becomes Peço-lho.

Why does the sentence use ao before the librarian?

The verb pedir follows the pattern pedir algo a alguém (to ask someone for something). In Portuguese, you literally say 'I ask the book to the librarian'. The preposition a (to) combines with the masculine article o (the) to form ao.

Why is the verb peço? Shouldn't it be more like 'pedo'?

The verb pedir (to ask for) is highly irregular in the 'eu' (I) form of the present tense, completely changing its stem to peço. Fortunately, the other present tense forms are more predictable, such as ele pede (he asks).

The English translation says 'ask for'. Why is there no word for 'for' in the Portuguese sentence?

The verb pedir inherently means 'to ask for' or 'to request'. English speakers often try to add an extra preposition like por or para right after the verb to translate 'for', but Portuguese doesn't need one here.