also: もんほん
noun
family crest register
A book or register containing family crests (家紋), used for reference or record-keeping.
古い紋本を調べて、自分の家の家紋を確認した。
I looked through an old family crest register and confirmed my family's crest.
家紋 is the family crest itself, while 紋本 is the register or book containing such crests.
Compound of 紋 (もん, 'crest') and 本 (ほん, 'book'). The reading もんぽん is a result of rendaku (sequential voicing).