I personally think it's nice to have some contributions by others, either full pages or decorations. I know I'm going to do some various decorated pages for my partner, along with other things.
Yay!! (Is all excited) *jump jump jump*
I'm also planning some pages that can be added to your existing collection among some other goodies, and I'm leaving margins so that they can be added to a book later without covering up any of the text or illumination. I've been feeling a bit like a monk in a scriptorium lately! It's been wonderful!! A glass of red wine, a bright flickering candle, a kitty taking a little nap on the nearby windowsill, fresh parchment---breaking in a new quill, that's going to be my Friday night--and probably my Saturday!
I personally like the thought of having a book with contributions from others. A lot of things, like the Wiccan Rede, charge of the Goddess etc, are pretty standardized texts as well, and look BOMB in calligraphy or a neat looking cursive script.
REMEMBER: Tomorrow's a full moon! A great time for crafting your goodies!!