in MCSE what number means ?!!

when i was looking for books about mcse ceritfication .. i noticed that each book has a number like 70-250 or 25-457 (the latter is wrong ;) ) . can anyone tell what do these numbers refer to .. maybe the quesion is selly so that i 'll ask another quesion . if i want to prepare myself to take the exam what do i have to do ? in other words is there one book that cover every aspect and valid to all exam like A+ certi. ... does each exam of the seven require a seperate book ???? i'm alittle confused :confused:

thank you all