EVERYTHING you ever wanted to know about birth, but were afraid to ask – plus a few things you never wanted to know – are explained with wit and experience in the useful and entertaining “The Big Book of Birth” by Erica Lyon (Plume, $15). Lyon is the local hero who founded the Realbirth center on West 22nd Street, and in this title she offers more than a decade of experience preparing parents-to-be, including proven techniques to speed up delivery and many options for managing pain. I only wish this reassuring, humane and endlessly useful handbook had been in print when my daughter was born.