I've had this happen frequently when exporting book files that are structurally complex (IE placed architectural drawings combined with multiple additional layers / transparency). I've tried John's method and found each individual file exports fine on its own, but as an entire book stops responding.
If each individual file exports fine on its own, try exporting the book in smaller groups of documents (how many depends on the number of pages and how complex they are), then combining in Acrobat.