"Van valami érdekes ebben a könyvben."

Translation: There is something interesting in this book.

[guilth]

is it common to start an affirmative sentence with "van"?

[vvsey]

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:
https://www.duolingo.com/comment/17228519

[Shamarth]

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.

[Imre565437]

Van valami érdekes erről a könyvről