In forums such as this, where it's not possible to format the title of a question, is there an appropriate way to refer to a title of a book? For example, in this question I enclose the title of a textbook in quotes. Is that okay?
Yes. For example, a typeset Chicago Manual citation:
can be represented in plain text as:
AP style allows underline (for historical reasons, because it is available on a manual typewriter). As StackExchange markdown does not support underline, I do not give an example.