Descriptive Books used to be crafted with a book and a feather:
In the latest version of Mystcraft descriptive books can only be crafted in a bookbinder using leather and paper.
When they are crafted, they will be empty. By putting them in a Writing Desk, one can add Symbols from a Notebook by clicking on the Symbol, but note that Symbols cannot be removed. The book can be traversed, similar to a Linking Book. Doing so will automatically generate any unspecified required Symbols, or generate an entire Age on its own, if no Symbols are specified. Each Descriptive Book will link to a different Age, even if the same Symbols are used. Descriptive Books can also be placed on Linkbook Stands.
NOTE: If the Desciptive Book is lying open on the ground, there is only one way to pick it up and put it in your inventory. You must right-click on the book, and then left-click the book icon on the top left and drag it into your inventory. Hitting a Descriptive book will only destroy it, not dropping the item.
NOTE: Visiting Mystcraft ages can create lag. This is due to the fact that once you return to the overworld or enter another age the previous age is still in the temporary memory. Simply restarting your client every two or three jumps between ages should return your fps to normal.