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