I don't think the OP has provided enough information to get a useful answer to his/her precise question (what text to learn quickly from).

What level is the course being taught at? High school? Undergraduate for non-majors? Undergraduate for majors but without specific knowledge of any other undergraduate math courses beyond calculus? Undergraduate assuming some basic analysis and/or algebra? Graduate level? Something else??

As others have said, a perfectly reasonable thing to do when you are teaching any course for the first time and don't have strong opinions / too much expertise about it is to look at the textbook(s) that others have used who have taught the course recently. Thumb through them a little bit, then ask them how they liked the book and how well it worked for the course. If you found anything confusing or problematic in the book, ask them about that.

I think someone with a PhD in mathematics (for the sake of argument, I'll assume the OP has one) should be able to pick up and read a textbook for any undergraduate class within a month and then be able to teach the class with a reasonable amount of competence. Of course, real insight takes more time than that, and it is not reasonable to expect that someone conscripted into service with one month's worth of notice (why is this, exactly?) will be able to provide that.