The Book of the Dead is a mechanic unique to Necromancers in Diablo 4. It allows them to summon minions to fight with them, as well as customize the minions.
Unlike the other Classes, Necromancers can start taking advantage of this mechanic at level 1 — they will have the Raise Skeleton Skill, allowing them to use Corpses to raise Skeletons to fight alongside them. At level 5, the Book of the Dead will be fully unlocked, and you will be able to start customizing your minions.
There are 3 tiers of minions you can summon: Skeletal Warriors, Skeletal Mages, and Golems. Once the tier is unlocked, there will be three versions of the specific minion to summon, which unlocks gradually while leveling — you can only choose one of these options to summon per tier. Within each option, you can either choose one of two Upgrades to enhance them, or choose the Sacrifice option, which disables your ability to summon that minion type in exchange for a passive buff.
Skeletal Warriors are available at level 1, while Skeletal Mages are unlocked at level 15. At level 25, you will get the quest Necromancer: Call of the Underworld. Once this quest is completed you will gain a Golem Skill, which will allow you to summon the Golem Minion.