There are many different books in the ' crazy silly school ' series. If you have read 41 of the books and are yet to read 18 books How many books are there in the ' crazy silly school ' series?
There are 41 books that the person has read. 
There are 18 books that the person has not read. 

That means that there are a total of 41 + 18 = 59 books in the series.