There are three main cycle maps covering Somerset. Sustans have their Somerset Levels cycle map, which is a pocket-sized map, and very clear. It covers the county as far as southern Bristol and across to Bath. If you need Bristol northwards, you need the Severn and Thames Sustrans cycle map.
The Devon and West Somerset Ordnance Survey Tour Map is great for planning, if you are cycling in the western part of Somerset and in Devon. A bit big to carry around, but I have been known to trim and take what I need!