GCSE level math books (end of Junior high, first year high school i think)

Aeolus

hi,

i would like to study some books about mathematics that clearly shows how something works and then some problems to practice on if possible. i am not in university or sixth form yet as you may guess (sixth form = last 2 years of high school i think) but just right under that, i really want to be excellent in mathematics instead of just good so therefore i want to know about some books which i would benefit from studying.