When soul speed was added it was said to only be collected through bartering or bastions. However, I have managed to obtain a soul speed 2 book through a raid. This is not ment to happen. 

How to reproduce:

1. Start a raid. (Make sure you have a looting 3 sword. It makes it easier.)
2. Kill all the raiders in a raid and if they drop enchacnted books check for soul speed.
3. This may take a while but I obtained it in my first 1.16 raid.