Here is one common way to categorize sentences:
simple
compound
complex
compound-complex
Simple sentences are covered in Book 4; compound sentences are covered in Book 5; most of Book 6 focuses on the complex sentence, along with a quick peek at the compound-complex sentence.
Our journey toward complex sentence mastery will be assisted by three key word groups: the subordinating conjunctions, the relative clauses, and the noun-clause markers.
We should note that the term complex is not really about complexity.
Example:
complex sentence: It's quiet when he leaves.
Yes, that is a complex sentence and, yes, complex sentences can be that simple.