besides mr 666 is not the number of the Devil, dont know what books you reading but none of the ones I know say that its the number of Julius Ceasar
here is the quote from the Bible that the number of the beast comes from
"And that no man might buy or sell, save he that had the mark, or the name of the beast, or the number of his name. Here is wisdom. Let him that hath understanding count the number of the beast: for it is the number of a man; and his number is Six hundred threescore and six." New King James Translation
a score is twenty, so 3 times twenty is 60 + 600 + 6 = 666
however some scholars debate whether this was merely code for various things, such as Nero or the of date "bad guys".
Regardless it is seen as a representation of evil