1. An excellent and detailed website from the Imperial War Museum about the Battle of the Somme, 1916.
IWM Somme website
2. Also well worth a read are some of the many personal stories from the Somme, including the experiences of JRR Tolkein. IWM Somme personal stories.
3. Check out the video clips on the WW1 'quick links' sections for some very good video clips about the Battle of the Somme.
4. This animated map from the BBC also gives an excellent overview of the Battle.