Extracted from the second thesaurus explanation in the Oxford Dictionary:
Understand or grasp?
You can use understand or grasp for the action of realizing the meaning or importance of something for the first time:
It's a difficult concept for children to understand/grasp.
Only understand can be used to talk about languages, words, or writing. So, this is incorrect:
I don't grasp French/the instructions.
With respect to the preceding definition, I am wondering if B is correct usage, where A is the intended meaning:
A.I do not understand the book
B.I do not grasp the book