When it comes to books, the word “monospace” refers to a certain kind of typeface. In this typeface, every character is the same width; in other words, each letter or number occupies precisely the same amount of space on a line of text. This contrasts with most other fonts, where characters can vary widely in width.
Because they make the text easier to scan and read, monospace typefaces are commonly used in code editors and terminals. But there’s no reason why monospace fonts should be restricted solely to programming contexts: They can also be useful for typesetting books.
Why? For one thing, monospaced faces are easy on the eye when reading long stretches of prose because all characters take up the same horizontal space on a line—whether letters or punctuation marks. Monospaced type can also help detect errors more readily by making them stand out more clearly (as you might expect from something that makes all alphanumeric symbols line up).
But there are disadvantages too: One is that distinguishing between uppercase I and lowercase L becomes much harder when using monospaced font than with almost any other familiar font family (including proportional ones). Also, some readers find monospaced fonts blocky-looking and old-fashioned compared with many sans-serif alternatives.
All things considered, though—and particularly if your book contains lots of code snippets—a case could certainly be made for employing a good-quality monospaced face for long blocks of text: It’ll make your work look distinctive while maximizing readability and potentially even minimizing mistakes—a win-win-win situation!