I know the "Succinctly" series doesn't go deep into a topic, but this book takes a step further by not even respecting its own title.
The correct title would be "C# to F# Succinctly".
The reason is that there isn't many explanations about "functional programming", in the general sense, in this book. Here, most things are explained in the F# context, which, as the author points, isn't a pure functional language.
Also, it goes great lengths to make code unreadable by avoiding proper variable names and using things like "hd" and "tl" (instead of, say, "user" and "remaining_users") and "racc" and "lacc" instead of "sum_right" and "sum_left" (although I could point that this seems like a functional programming thing).
So, again: Want to use your knowledge of imperative programming to learn functional programming? Sorry, not this book. Know C# and want to learn F#? Then you have something to read.