In his newest Weekend Adviser column, Robert Hughes looks at the latest generation of pop-up books. With Harry Potter's final volume now old news, publishers in the $2.5 billion children's book industry are trying new tricks. They are putting out vintage-looking picture books that are packed with extras such as cards, maps, letters and even a facsimile X-ray plate or model ship.

On Tuesday, Candlewick Press publishes MYTHOLOGY (pictured to the right) the latest in a series that began with DRAGONOLOGY in 2003 and continued with PIRATEOLOGY, EGYPTOLOGY and WIZARDOLOGY. The titles and their spinoffs have sold more than six million copies, according to Karen Lotz, president and publisher, and the series has inspired imitators. Candlewick and others now refer to the entire genre as "ology" books, which are supposedly written by writers with Victorian-sounding names but are actually penned by series editor Dugald Steer.

These titles, Hughes says, can cost two to three times as much to produce as a standard picture book, yet typically are priced at $20, just $5 more. So publishers need to sell them in volume to make a profit. MYTHOLOGY has announced a first printing of 400,000 - a gamble in light of other titles, like BEATRIX POTTER: A JOURNAL (which tied into the movie starring Renee Zellweger) not doing so well. But if it does, then like other enhanced picture books, it can sell in a standard and steady way.