For a different twist, suppose that there is a special brand of necromancy at work in this particular tomb or your whole campaign that has origins in divine lore (perhaps the necromancer at work here worships a particular deity that has granted it limited powers to imbue undead with some divine spell casting power), and select individuals of the dwarf skeletons can cast one or two cleric cantrips. Approximately three of them would be roughly equivalent to four book skeletons, so CR 1/2 rather than 1/4.
They'd carry dwarf weapons like the mace, battle axe, warhammer, doing 1d8 +3 instead of 1d6 + 2 damage. I'd maybe do dwarf skeletons with 16 CON and 12 STR, that wear more complete armor (AC 14 or 15). Just be careful not to imbalance things or pose a quandry the party isn't capable of solving or surviving. It's always good to give your monsters some twists that players won't know from reading the MM. (Assuming a homebrew campaign) You are the DM. Phandelver and Below: The Shattered Obelisk
Mordenkainen Presents: Monsters of the Multiverse