The "every book possible" statement is an inference made by Superman, a logical conclusion he somehow reached from being told there were an "infinite amount of pages occupying the same space". He could be wrong, but ultimately it makes no difference. The idea of infinity through, what I can only describe as some sort of Copenhagen interpretation, is endless; it's not a real number; although we can and do use it like a number. Doing so enables us to expound on the idea of infinity and its properties. The statement ∞ > X > -∞, where X represents a real number, proves that infinity is greater than any X, whereas "negative infinity" (i.e. the negation of positive infinity and or the most negative value; -(2^(N-1))) is lesser than any X, demonstrates this.
Now, can something truly be endless? Yes. Euclid proved an infinitude of prime numbers:
Suppose that p1=2 < p2 = 3 < ... < pr are all of the primes. Let P = p1p2...pr+1 and let p be a prime dividing P; then p can not be any of p1, p2, ..., pr, otherwise p would divide the difference P-p1p2...pr=1, which is impossible. So this prime p is still another prime, and p1, p2, ..., pr would not be all of the primes.
Imagine a book with a single prime number, written once on a thousand pages made of lead. Now imagine a book where that same prime number is written twice on every page. Three times. Four times. How about that prime number stretched so only parts of the number exist on each page? What about one book where its font size is 0.15 and another where it's 923.45? Two prime numbers of varying fonts, three with varying fonts with different color inks, four with varying fonts with different color inks and inconsistent underlines for emphasis--the combinations are literally endless.