Resizing sounds trivial until you do it forty times for one book. This tool takes a batch of images, applies the same target to all of them, and gives them back without you opening an editor once.
The distinction that trips people up is between pixel dimensions and DPI. Pixels are the only thing actually stored in the file. DPI is a note attached to it saying how large those pixels should print. An image of 1800 by 2700 pixels is a 6 by 9 inch page at 300 DPI and an 18 by 27 inch poster at 100 DPI, and nothing about the file changed in between.
So when a printer asks for 300 DPI, what it needs is enough pixels for the physical size you intend. Work out the inches first, multiply by 300, and resize to that pixel count.
Pixel dimensions are the actual data in the file. DPI is metadata describing how large those pixels are meant to print. Changing DPI alone does not add or remove a single pixel; only resizing does.
Multiply the physical size you want in inches by the resolution your printer expects. A 6 by 9 inch page at 300 DPI needs 1800 by 2700 pixels.
There was no extra detail to enlarge. Software can only interpolate between existing pixels, which smooths rather than sharpens. Start from a higher-resolution original.