This is a list of esolangs you can disguise as regular literature:

• Brainfuck. The symbols are quite obscure, but you might be able to hide them in a math textbook. (Brainfork also works, although you have to make sure you don't put a Y in regular prose or as a mathematical variable (use lowercase instead)).
• .Gertrude. While you will have to get the right word lengths, you can disguise this language as almost any style of writing.
• ???. A variant of brainfuck with more common symbols. You can hide it almost anywhere, although it can be tricky to put the hyphen for a left arrow (hyphens aren't THAT common), and sometimes long strings of the same command can be tricky (I often end up doing something like "too bad, too bad, too bad, too bad, too bad, too bad" which gets rather repetitive (I actually "wrote" (generated automatically as brainfuck, translated by hand to ???, then put words between the punctuation) a program which doesn't do this, by relying on really long sentences)). You could of course balance it out with a no-op like +-, ><, or something more complex like +>-+<-.
• Half-Broken Car in Heavy Traffic. A quick skim through the standard compiler/interpreter source code reveals that invalid commands are treated as no-ops. Angle brackets and carets are a bit weird to use outside of a math textbook, and you'll have to restrict usage of the lowercase letters v and o (o can only be used once), but you can hide it in something. The 2d is a bit weird to manage, but the way the language is designed means that inserting empty rows or columns doesn't effect the semantics as long as the entire program gets those insertions except for @intext and @outtext. Invalid commands are treated like whitespace, so you can space the programs just fine.
• Grass. With strategic placement of W, w, and v, you can hide it in regular literature.
• Qdeql. The symbols are a bit odd, but it can be done possibly outside of a math textbook (backslash is probably the hardest, although you might be able to get away with it by using it in place of a forward slash.