Each player can carry one tome and one grimoire. Most of the main missions have 3 tomes and 2 grimoires. I highly recommend to collect the tomes and grimoires with a at least 3 players because it is tricky that the bots pick up a tome and keep it until the end of the mission.
A Tome takes the slot for your healing item. Tomes can be dropped and picked up by other players. Tomes and grimoires increase the chance of a higher roll at Ranald's Bones. The more tomes and grimoires you carry to the end of a mission, the higher the chance of a higher litter.
For the tome achievements you have to carry all tomes in one run to the end of a mission to obtain the tome achievement for this mission.
PS: I played and finished this game with a different gamertag -> "Achievement not yet won" display