Terrain can be made from anything... So, the way we'll describe you today. may be a little brutal, but it's definitely very interesting - book carving.
In fact, this art doesn't have much in common with wargames. This was first created by the American artist Robert The and quickly gained popularity, no matter how many book lovers protested against it.
To be honest - not all books are equally valuable. You always have some old telephone books or guides which are completely useless. Also you can use a pile of old papers for making such sculptures.
But this can make your wargame table really special and atmospheric! So, why not to use this idea? You can leave the books to show or imitate a rock with no trace of its origins.
It's difficult. it takes much time, but all the hobbysts are used to it. Moreover, such sculpture can be a nice base for the diorama. Don't be afraid to find inspiration in unusual sources!