Baby/Papa/Mama/Big Rudin

Recently, I was looking for the reviews of some Analysis books while encountered terms such as Baby/Papa/Mama/Big Rudin. Firstly, I thought that these are the names of a book! But it turned out that these are some nick names used for the books of Walter Rudin. So I was thinking that

$1$. What are the corresponding books of these nick names?
$2$. Why such nick names are chosen? or What are their origins?

Solution 1:

In order to sum up the above comments, the corresponding books for these nick names are

$1$. Baby = Principles of Mathematical Analysis;

$2$. Papa/Big = Real and Complex Analysis;

$3$. Grandpa = Functional Analysis;

and it seems that the difficulty of contents of the books grows with the age of the nick names! Firstly, you are a baby and things are easy to handle. Then you grow up and become a papa and things get more complicated. Finally, when you are a grandpa you should take care of your legacy very carefully which needs a hardwork! So $1$ is a prerequisite of $2$ and $2$ is prerequisite of $3$.