Are they mostly sought after, because they can turn into tomes?

The short and only answer: yes. There is also an achievement for having all the amulets in wardrobe, but that's a pretty insignificant usage compared to turning them in for tomes. ...or mainly shards, but anyway.

(Also, love your sig for the obvious reasons ^^)

, thanks for the answerc: Praise Besus! Someone who actually knows who it is and doesn't think it's Eddie Redmayne's stunt doublecx