Finding errors in a large book

As Phelype says the error is on line 171 (almost certainly the last line) of PaPCh11.aux as that file is re-created every time you run latex, you can delete it and just run latex again.

By far the most common way to corrupt an aux file leaving a partly written command is to kill the latex job either by a key shortcut such as control-c or by killing the terminal window (x icon at top right in Windows) If you get a tex error, allow tex to finish cleanly and complete any file writing by typing x to the ? error prompt. If you kill the process any files currently open for writing may get corrupted with unfinished lines (due to system dependent cached file writing out of the control of TeX).

Note that TeX logs a ( and the file name when it inputs a file and ) when the end of file is reached so

! Undefined control sequence.
l.171 \cit

unambiguously points to line 171 of PaPCh11.aux

I have another thought. Did you mean that if you search for \cit, the editor will show you \cite together which you don't want. However when you write \cite, it often follows a par of curl bracket {...}. Then you can just search for \cit{ in your source file to find this error.