Books

Exploring the World of the Celts by Simon James (Thames and Hudson, 1993)
Recounts the history of the ancient Celts, notably the Catuvellaunians who dominated the region of what is now Hertfordshire and who probably drove the Atrebates to the south. This unstable situation led to the Atrebates seeking help from Rome, and the subsequent invasion of Britain by Claudius.
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A History of Roman Britain by Peter Salway (Oxford Paperbacks, 2001)
From the invasions of Julius Caesar to the unexpected end of Roman rule in the early fifth century AD, this is an authoritative and comprehensive account of Roman Britain.
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Iron Age Britain by Barry Cunliffe (B T Batsford, 2004)
Looks at social change within the insular society of late Iron-Age Britain, which was ultimately dominated by the emergence of Rome as a mega-state.
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Roman Britain by John Wacher (Sutton Publishing, 2001)
An introduction to this period looks at life before the Romans arrived and then the massive changes imposed on the native peoples of Britain following the Roman invasion.
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Links

De Imperatoribus Romanis – Claudius 41-54 A.D.
www.roman-emperors.org/claudius.htm#Note%1
Emperor Claudius initiated the full-scale annexation of Britain as a province, which remains today the most closely studied corner of the Roman empire.

The Landings of Caesar in Britain, 55 and 54 BC
www.athenapub.com/caesar1.htm
Biographical information about the Roman Emperor Julius Caesar who led the two invasions of Britain in 55 and 54 BC.

Roman Perceptions of Britain
http://penelope.uchicago.edu/~grout/encyclopaedia_romana/britannia/miscellanea/geography.html
Interesting article from 'Encyclopaedia Romana' that looks at why the Romans were so determined to conquer Britain.