ShareX, free tool for Windows.
https://getsharex.com/
Incredibly powerful, you can define actions and "workflows" that are like an automated sequence of steps that occur when you capture an image (open it an editor, or save it, or copy to clipboard, or upload it to imgur/other places, or some combination of these). You can let it take over your PrtScrn key on your system, and you can define shortcuts for example use Ctrl + PrtScrn to capture regions, Alt + PrtSrn to capture active app window, etc. You can also predefine the SIZE of a region capture that you want, for the case the OP describes, repeatedly capturing the same region as you flip pages. Of course, you could also just capture active app window with something like ALT + PrtScrn, would have the same result except it'd include the app menus/icons of whatever reader app you're using.
The UI takes a little getting used to, but it's the most powerful screen capture, lightweight image editor, file uploader, etc., utility that I have tried. I have it now so when I do a screen catpure with CTRL + PrtScrn, it lets me grab a region, when done, it copies to clipboard, saves to my cloud account, and also uploads to my imgur account. All automated.