The C# Player’s Guide now has expansions! These are small additional content in book-like form (PDFs on Gumroad) that are generally free (but with an option to pay if you feel so inclined) additions to the main book.
The expansions are not meant to be stand-alone. They’re meant to augment the book. They’ll often refer back to sections in the main book for more information, and some of the challenges will also refer to or build upon the challenges in the main book.
5th Edition Expansions
C# 11 Expansion
This free 18-page expansion covers the new C# 11 features that were released in November 2022.
Available on Gumroad: https://rbwhitaker.gumroad.com/l/csharp11expansion
4th Edition Expansions
The fourth edition does not have any formal expansions. However, the fourth and fifth editions are similar enough that you should be able to leverage the fifth edition expansions without any trouble.
Additionally, the Upgrade Companion could be thought of as a C# 10 Expansion, which can be found here: Upgrade Companion for 4th Edition.