If you’re doing everything in the Command Window, there is no going back.
I would open and save a script file for your exercises and run the script file instead. You can Comment (use % in the first column) the lines you want to save but do not want to run for later reference as you go through your textbook. (I have a script file like that to test and save code I post here on Answers.) You can also use comments to refer to specific exercises on specific pages with respect to your code, so you can refer to it later.