When importing a skeletal mesh there is an issue where the normals of the face do not update with the faces of the inside of the book as it opens. This can cause a noticeable shadow issue, which could be more content related as well. However, looking at the normals in comparison to 3Ds Max and UE4, they are not updated, even using the Import Normals option.
I've attached the asset and FBX and Max File if needed.
1. Open UE4
2. Import Test Asset
3. Open Animation and enable Normals view
Results: Normals for the crease of the book do not update with the faces of the book.
Expected: I would expect there to be a normal for each face and update like in 3Ds Max when looking at the normals.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-26894 in the post.