Right now there is no one good book on Iron Age helmets in the
oikumene, just a few with different focuses and blindspots.
Reallexikon der Assyriologie s.v. Helm: Archäologisch (horsehair helmet crests seem to have been invented by the Neo-Hittites then spread to the Assyrians and the Aegean)
Eero Jarva's
Archaiologia on Archaic Greek Body ArmourConnolly's
Greece and Rome At War (just keep in mind that a lot of his 'classical Greek' kit is from sites in Italy)
Encyclopedia Iranica s.v. Helmet (i)
https://iranicaonline.org/articles/helmet-iDintisis'
Hellenistische Helme (2 volumes)
Bishop and Coulston's
Roman Military EquipmentHelmets still have horsehair crests in the Strategikon of Maurice and
a silver Lombard bowl.
Early and high medieval Europe are not my specialty, but by the 13th century the Franks have those
cuir boille decorations for the helm, and they last into the 15th century. They serve the same function of making you look taller and standing out in a sea of screaming moving bodies and flashing weapons. Codex Manesse and the Innsbrucker Wappenrolle and other MS interested in heraldry tend to show them alongside the shield
http://codicon.digitale-sammlungen.de/inventiconCod.icon.%20312%20c.html By the trecento we also have bascinets fitted with "a little tube of silver to carry the plume" for a few soldi extra.
Claude Blair and David Nicolle are good 'go tos' for medieval arms and armour.