For the Bookshelf,

These two are absolute Must reads.

Pilgrim Souvenirs and Secular Badges

Brian Spencer, Museum of London

latest Boydell reprint, ISBN 978 184383 544 8

(Spencer MoL PSSB)

Dress Accessories

Egan & Pritchard, Museum of London

ISBN 0 85115 839 0

also worth having, 

Salisbury Museum Medieval catalogue part 2

Brian Spencer;

Pilgrim Souvenirs and Secular Badges

ISBN 0 94753 512 8

Medieval Chic in Metal
decorative mounts on belts and purses from the low countries, 1300-1600

Annemarieke Willemsen & Marlieke Ernst
ISBN 978 90 8932 111 4 

And

On Divers Arts

Theophilus

Tr. C.S.Smith, Dover Reprint.

ISBN 978 048623 784 8

Online;

The most active Academic researcher at the moment is Dr Daniel Berger.

He publishes most of his work online, look for 

"Steingussformen aus dem spätromanischen-frühgotischen Magdeburg" and go from there.

http://www.academia.edu/687683/Steingussformen_aus_dem_sp%C3%A4tromanischen-fr%C3%BChgotischen_Magdeburg._Arch%C3%A4ometrische_und_experimentalarch%C3%A4ologische_Untersuchungen_zum_mittelalterlichen_Zinnguss_an_ausgew%C3%A4hlten_Fundst%C3%BCcken

His work is all in german, but still worth a look at for the illustrations alone, particularly the cat scans of extant moulds showing all the sub surface work. (Air voids, Locator pins etc.)

and Wonder of Wonders; The Religious and Profane Medieval Badges Foundation

http://www.medievalbadges.org/mb_index_UK.php

Related Arts:

For Seals,

A Guide to British Medieval Seals

Harvey & McGuinness

ISBN 0 7123 0410 X