"Themselves" or "Themself" in a singular context? [duplicate]

The singular "they" and its spawn are notoriously frustrating and controversial. Because your question is about what you "should" do, I would suggest a reasonable workaround:

In order to read the remainder of this book, the reader is advised to become familiar with the concepts contained within this Chapter.

I believe your understanding of what is acceptable, in terms of the reflexive pronoun, is correct. However, in most cases it is relatively easy to sidestep the conundrum altogether.

The word you want is himself. Himself is the reflexive pronoun for someone of unknown sex.

Try: To read this book, the reader should learn the concepts contained within this Chapter.

Try: To read the rest of this book, the reader should familiarize herself with this chapter.

Try: Before reading further, you should learn this chapter thoroughly.