Motifs are books mandatory for crafting styles that aren't from your starting race.
Motifs are purely cosmetic, but worth farming if they're rare or expensive enough. They often drop in value extremely fast after they've been released, so only a few are worth farming for.
The recial motifs are the least rare, on the exception of the imperial one. And some are rare but not extremely expensive anymore, like Daedric Motif and Ancient Elf Motif.
The Racial motifs
There's also style motifs, most of them found in dlc areas, bit some can be found in the Crafting Writs or Daily Quests, like Glass Motif Fragments and the mercenary motif
Thieves guild DLC motifs
Orsinium DLC Motifs
Dark Brotherhood DLC Motifs
Clock Work City DLC Motifs