The National Archives is a good place to start
http://www.nationalarchives.gov.uk/records/research-guides/british-army-tracing-soldiers.htmBooks:
The History of the Boer War; Volume 2 - Foster Hugh Egerton Cunliffe, 1904
Cassell's History of the Boer War; 1899-1902 Vol 1 - Richard Danes, 1903
Steady the Buffs! A Regiment, a region, and the Great War - Mark Connelly 2006
The 3rd Battalion "The Buffs" In South Africa, H. D. Hirst 1908, published by H. J. Goulden
Historical Records of the Buffs, East Kent Regiment (3rd Foot) Formerly designated the Holland Regiment and Prince George of Denmark’s Regiment, by Charles Raleigh Bruere Knight. 1935.
A short history of the Buffs, East Kent Regiment (3rd Foot) formerly – Eric Foster Hall 1929
Simon Fowler and William Spencer, Army Records for Family Historians (2nd edn, PRO Readers' Guide, II, 1998)