Remove the book jacket from the book. Lay the creased section of the jacket flat on a table. Allow the spine and opposite cover of the jacket to hang off the table. Place the book squarely over top of the crease. If the book is not especially weighty, add more books to increase the pressure. Be certain that you aren't creating any new creases, or removing any creases that are actually necessary.
Wait a day and then lift the weights off the book jacket. Inspect the former crease to see if more pressing is necessary. If the crease is satisfactorily removed, place it back on the book. If not, return the book jacket underneath the weights.
Fold the crease gently in the opposite direction if pressing doesn't appear to work. Some jacket materials may be resistant to pressure and need have their creases removed manually.