Stop payment on all the checks -- tell your bank your checkbook was taken and you very well may NOT have to pay for the stop payments. If the checks have already been presented this will also work.

File charges against the person who STOLE your checkbook. Besides theft, they're facing forgery charges -- and possibly identity theft, defrauding a merchant, and other charges (depends on your state). Do this whether or not you consider them a friend or even a relative -- they've proven that they are no friend of yours.