In my opinion, both sentences are absolutely correct, but there is a difference in connotation. I’ll start with the second sentence.
I find it hard to understand this book.
For me, this is a general statement of fact or opinion, with no feeling of time involved. (e.g. I think it is hard to understand this book. No matter how many times I look at it, it’s always hard to understand.)
But, you could also use this sentence to talk about a current (first time) reading of the book. The verb ‘find’ can indicate an opinion. Verbs of “opinion” often/usually are not used in the continuous form at all. For example, you should not say “I am thinking this is an interesting book.” You should say “I think this is an interesting book” — even if you’re currently in the middle of reading the book for the first time.
So, I find your second sentence to be pretty standard.
I am finding it hard to understand this book.
This sentence clearly refers only to a current, on-going situation and emphasizes that. For me, it indicates that your difficulties in understanding have just recently begun (because you’ve just recently started reading the book). Possibly you also want to imply or emphasize that the difficulties are “currently in the process of growing or developing”.
That’s my gut feeling. Hope it helps.