Learn Swedish: Jag har boken.

The infinitive is ha (to have). It is a very common irregular short verb. Instead of adding a longer ending like -ar or -er, it simply adds an -r directly to the final vowel to make the present tense, becoming har.

Where is the word for "the" in this sentence?

In Swedish, you usually don't use a separate word for "the" before the noun. Instead, it attaches to the end of the word as a suffix. En bok means "a book", and adding the -en suffix turns it into boken ("the book").

Why does boken end in -en instead of -et?

It depends on the noun's gender! Because the word for book is en bok (an en-word), it takes the -en ending. If it were an ett-word, like ett hus (a house), you would add -et to get huset ("the house").

Is the "g" in jag pronounced?

Sometimes, but rarely! In everyday spoken Swedish, the "g" in jag is almost always silent, making the word sound exactly like the English word "ya".