"Angular Harps Through the Ages, A Causal History" by Bo Lawergren. A scholarly, illustrated article about the angular harp 3000 B.C. to 1600 A.D
See also his article on angular harps in the summer issue (2011) of Early Music America.

Celtic Harp - From ancient times to the present.

Chromatic Harp

Early Gaelic Harp Info - the historical Gaelic harp tradition of Ireland and the Highlands and Islands of Scotland.

Early Welsh Music and Instruments - medieval and renaissance Welsh music and poetry with voice, crwth and lyre.

Early, Medieval, Celtic Harps in three chapters.

Gaelic Harps and Harpers in Ireland and Scotland

Hair Raising Harp History: Deborah Henson-Conant asks questions, Nancy Thym-Hochrein answers.

Harp History

Harp History and Culture - A listing of book titles.

The Harp: History, Care and Feeding of

The Harp in Wales

History of the Harp in Scotland

The kora (French: cora) -- the 21 string harp-lute played by Mandingo peoples throughout West Africa.

"Nicolas-Charles Bochsa: harpiste, compositeur, escroc (harpist, composer, crook)", new biography (in French) by Michel Faul.

Small Harp History Since 1800 - Evolution of the small harp design.

The Wire-Strung Harp - Excerpts from Rensch, Rimmer, and other researchers.

Wikipedia: harp history, types, world harps.