Cheatsheet is in-game library for class spells (or cross-class spellbook).

It contains baseline spells, passives, talents and honor talents and even artifact perks including those added in 7.2 patch. Spells can be searched by name or browsed by class and specialization.

Cheatsheet's purpose is to be a simple, fast way check on abilities other classes have without leaving game or having to look in several places for basic tooltip information.

Includes:

Spellbook (baseline) spells for each specialization
Talents and Honor talents.
Artifact traits and artifact ability.
Search function for finding spells based on their name

Commands:

/cheat or /cheatsheet to open.
/cheat config or /cheatsheet config for options.
Left-click spells to "mark" them, right-click to remove marking.
Shift-click spells to link them to chat.
Control-click spells to display their SpellID.

Known Issues:

Some talents have different effects on different specializations and therefore different tooltips. This causes some talents to display weird tooltips (usually the case with talents shared between specializations) I cannot do much about this as it is intended behavior by Blizzard.
Artifact traits with multiple ranks always show tooltip for rank 1.
Due to how this addon is made it will break every time SpellIDs are changed, which happens often with bigger patches.

Recommended:

PvPTooltips (PvP spell modifiers on tooltips)
NameplateAuras