Learn Turkish: Hobisi kitap okumak. - Her hobby is reading books.

The Turkish word for hobby is hobi. To say "his" or "her" hobby, we add the 3rd person singular possessive suffix. Because hobi ends in a vowel, we must use the buffer consonant s before adding the suffix vowel i. This gives us hobisi.

Why isn't there a word like onun for "her" in this sentence?

The possessive suffix -si in hobisi already tells us the hobby belongs to a third person (his, her, or its). You can include onun at the beginning for emphasis (onun hobisi), but it is usually dropped in everyday Turkish because the suffix does the job.

The translation says "books", so why isn't kitap plural here?

When talking about a general category or activity, Turkish uses the singular form of the noun. So the activity is literally "book reading" (kitap okumak). We translate it as "reading books" because that sounds more natural in English.

Why does the verb okumak end in -mak here instead of having a personal suffix?

okumak is the dictionary form, or infinitive, of the verb. It translates to "to read" or "reading". Here, it acts as a noun describing the activity itself. The sentence literally means "Her hobby is the act of reading."