https://mastodon.gamedev.place/users/havchr/statuses/116185423094393534
I'm applying the same animation matrix to the normals (with 0.0 as w component of the vec4 as this is a vector not a point).
Upon closer inspection the main problem seems to be in the fold of the book. Here both joints of the opening halfs are applied. Maybe that causes problems as the cancel each other out.