In my world, it is a special book that has personal information inside it. It might be secrets of one's heart or knowledge of things that one has found out. It could be spells, secret language, or any other thing that is important to the owner.

Also, in my world the grimoires are able to be spelled meaning that depending on the spell or spell set, if someone other the owner tries to open it or sometimes even touch it, the grimoire will react accordingly. 

The owner can place grimoire creatures inside to help the owner. More on that later.