There used to be two primary book formats: hardcover and paperback. (bngobooks.com)
X-Men: Battle of the Atom (Marvel Entertainment): This big, fat hardcover collects the ten - part, old - school crossover of four X-Men monthly titles, which was book - ended by X-Men: Battle of The Atom # 1 and # 2, and ran through two issues each of the Brian Michael Bendis - written primary books of the franchise (All - New X-Men and Uncanny X-Men), and the secondary titles (Wolverine and the X-Men and plain old X-Men, written by Jason Aaron and Brian Wood, respectively). (cbr.com)
On conservative estimate, if only 5 % of one year's primary book sales wound up in the secondary market annually, that is 70 million books a year4 being resold. (michaelgracie.com)