I am a blender newbie and I'm trying to animate a storybook opening, a few of the pages turning, and then the book closing. I have the animation keyframed, and I have 2D jpeg illustrations that I have placed on the flipping pages, but when added to the Plane models for the pages the 2D jpegs are being stretched (as seen in the photo). I've viewed a lot of tutorials trying to figure out how to fix this, but without any luck unfortunately. If anyone could please give me some guidance or troubleshoot with me, I would greatly appreciate it.
In general you choose quite difficult topic for beginner ...
First problem - wrong UVmap
... fix - "Project from View"
BTW: As you can see your image is different proportion than the page.
Second ... you are stretching page geometry ... your page is not sliding, but is stretch by modifier = it stretching UVmap too.
For cover I use lovely addon Convert Mesh to Armature ... by one click it generates rig for you (it starts hierarchy from selected face, so select right side before running operator).
For deforming page is usually used animated curve object (animated via shape key) ... this object is used for Page object with Curve modifier.
You can try to rig pages - Armature with bone type Bendy Bones.