There are 11 phases (levels) in the main story mode. Each of the difficulty settings count for 33% of story progression, so by the time you complete the "Brutal" setting, you will have just about completed the story percentage.
After that, there are the survival modes which pit you against increasingly stronger waves of enemies. The only way to complete Jen's album is to finish these levels. And the only way to complete Dr. West's journal (I believe) is to beat the story mode on the Brutal setting.
I too am enjoying this game. I think it has been completely overlooked because of the subject matter. Taken for what it is, Splatterhouse is a fun, highly enjoyable, and quality beat-em-up. It doesn't break any new ground, but it does give us an awesome re-imagining of Rick and Jennifer's original exploits in the West Mansion. And that is a trip I am glad I took again :)