They aren't, and I would go as far as saying that most users should never interact with their raw private keys unless they really know what they are doing and they have a specific reason for doing so. Trying to extract individual private keys from a wallet for the purpose of back up will likely lead to exposing those private keys and risking the coins contained, or not backing up enough information to fully recover all the coins in the wallet. Most users should just back up their seed phrase and forget about even looking at their private keys.

More basically than the answers above: Hexadecimal is a counting system. Hexadecimal numbers are, well, numbers. We simply use the characters A-F to refer to the numbers we more commonly think of as 10-15, because they are characters that are distinct and familiar to everyone. They are not being used as letters, simply as recognizable characters.

I've never noticed before, but Mastering Bitcoin is actually wrong here. 0 is not a valid private key.