GIAC is a unique certification provider in that there is almost never third-party training published. The primary and BEST way to prepare for a GIAC certification is by attending the appropriate SANS training course. SANS courses provide all of the training material you need to pass the certification exam and help to reduce the preparation and study time required.
SANS courses tend to be fairly expensive if you are trying to self study, however there are other vendors that provide similar certifications for less.