The Tome of Strahd is an ancient work penned by Strahd that explains how he came to be what he is. The Tome is very important to Strahd.
There is a lock on the clasp of the tome that one of Diath's mysterious keys was able to open.
When the waffle crew traveled into Barovia's past, they showed Tatyana the Tome of Strahd to convince her that she was in danger. They later showed it to Rahadin, who kept it.