The book of public arms : a complete encyclopædia of all royal, territorial, municipal, corporate, official, and impersonal arms by A.C. FOX-DAVIES; T.C. and E.C Jack; London 1915
This is the essential reference for all arms that belong to entities other than families. There is a brief introduction and then a simple alphabetical list of entries, the majority with clear illustrations. Available from archive.org in a text-searchable PDF and other formats at http://archive.org/details/bookofpublicarms00foxd.
Good scan quality, almost all pages clearly readable, image quality good to excellent. OCR accuracy 95%+
NOTES: The edition linked to here is of excellent quality but is missing pages 761 and 762. These contain entries for The Royal College of Surgeons, Surrey, Sussex West and Sussex. There is an alternative version of the same work at http://archive.org/details/cu31924029798927 which is not of such good quality (centre of double page spreads obscured in places) but it does contain the missing pages.