A Look at Monster Book

The total displayed in your character info is only UNIQUE monsters. If there are 8 monsters in Maple Island then it would say you had 8 cards, and never any more.
It displays 3 numbers:
Total number of unique cards (this is the sum of the next two)
Total number of unique regular cards
Total number of unique special cards

So, for example, if I have 5 king slime cards, and 5 Deu cards, but if you look at my special card total it only says 2, not 10.

OH! Maybe that factors the monster book level, too... I mean, maybe 5 from the same count as 5 different ones in that case.