Learn Swedish: Boken är tung. - The book is heavy.

Because bok is an en-word (en bok). When an adjective describes a singular en-word, it stays in its base form. If we were describing an ett-word, we would add a -t to make it tungt.

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

In Swedish, "the" is usually attached to the end of the noun rather than being a separate word. The base word is bok (book), and adding the -en suffix gives us boken (the book).

Does the adjective change because we are saying "the book" instead of "a book"?

Not in this type of sentence! When an adjective comes after the verb är (is) to describe a noun, it only changes based on whether the noun is an en-word, an ett-word, or plural. Since boken is a singular en-word, tung stays in its base form.