Gods and Worshippers: In the Viking and Germanic World by Thor Ewing, Paperback | Barnes & Noble®
Thor Ewing on Twitter: "@CatJarman I don't know of anything directly comparable. In 'Viking Clothing' (p106-7, p109) I suggest these 'knappar' might not be buttons-as-clasps but purely decorative fittings from a 'slæðr'