Eh, there's no summer reading persay. They ease you into it really gently as biology is not a requirement at A Level, but the first term is almost entirely A Level biology based.
If you're desperate, Essential Cell Biology is the book which Molecules, Cells and Disease is based on - but there are copies in the library + it's a monster of a book to just 'read'.
Honestly, have fun and enjoy the summer. You won't be at any disadvantage by not doing any reading, unless you didn't do biology A level, in which case I recommend you get Essential Cell biology and work your way through it.