If you're writing a series, it won't take long to find you've forgotten some of the details from your earlier books. What car did she drive again? Did he rent an apartment or buy a condo? Did she have a political science degree or an economics degree?
Starting a story bible from the beginning – or at least starting it now – will help you remember what happened when, to whom, and whose cousin got in trouble for it.
I go over some of the basic things you should keep in your story bible as well as more detailed information. If you want more ideas on how to get started, one of my favorite articles on building a story bible is this old one from the Fantasy, Futuristic & Paranormal chapter of Romance Writers of America. I no longer use paper and binder (as this article suggested), but I do pretty much the same thing using subdocuments in a Scrivener file.
Here is another take on it from the same group. In fact, if you search for “series bible” or “book bible” or “story bible,” you will find a lot of great advice from different perspectives. Good luck!