A Byte of Python Swaroop C H's online book offers many, many little knowledge chunks. Each page offers a tiny bit of Python knowledge, with clean formatting and good examples.

A Byte of Python is nominally available for free as a web-based book, but is also available as a PDF download and in printed hard copies. Many translations to languages other than English also exist.

In Python 2, you don't need to talk about byte strings explicitly, but by using them, you can make it very clear as to whether you're using bytes or characters.