is it common to start an affirmative sentence with "van"?
Yes, it is quite common. For example:
"I have two tickets for the game." - "Van két jegyem a meccsre."
It is not just with "van" but with any verb really. In fact, with any predicate. Here is an extensive discussion on the topic:
I wonder if this could be translated as "There is something interesting about this book". At least translating it back into Hungarian we would use the same structure with -ban.
Van valami érdekes erről a könyvről
Not to still be asking basic questions 250 days into the gruelling torture that is Hungarian Duolingo, but how do we know it is "this" book instead of "the" book in this sentence?
ebben a könyvben = in this book
a könyvben = in the book
look out for "ebben", which is ez+ben