Learn Dutch: Lees verder in dat boek. - Read further in that book.

Lees is the command form (the imperative) of the verb lezen (to read). In Dutch, you tell someone to do something by using the stem of the verb (the ik form) and putting it at the very beginning of the sentence.

The full verb is lezen with a Z. Why does it change to an S in lees?

Dutch words never end in a Z or a V. To get the stem of lezen, we drop the -en and change the Z to an S. We also double the E to keep the vowel sound long!

What role does verder play in this sentence?

Verder means further. When paired with a verb, it means to continue doing that action. So lees verder means to continue reading or to read on.

Why is it dat boek instead of die boek?

Because boek is a het word (het boek). When saying "that" for a het word, you must use dat. You only use die for de words.