Lock from scaling?


I’m trying to figure out if there is a way to lock certain parts of a mesh from scaling.

See attachement for clarification.

I’m trying to (first blender project - great fun btw) create an animation of books tipping eachother over, like a domino.
I created a base mesh of a hardcover book and scanned (tedios!) about 40 covers. UV mapping is done and texture files created.
Now I need to scale the books to proportion.
Height (Z in my setup) and width (X) aren’t a problem, I just use a box select and move it along the Z or X axis.
It’s the Y axis that’s giving me some troubles.

I’d like to scale it on this axis without having the cover itself scale.
So is there a way to lock certain vertices and the distances in between, and still be able to move them/scale the rest?

I hope I’m making at least some sense. Feel free to ask more info if needed.