I would recommend first thing would be to get out on the manufacturing floor and observe what is going on. Talk to the workers and the supervisors as to what they consider problem areas (you may not get the same answers). There are many potential causes for high WIP from physical to political so spend some time evaluating the information you gather before starting efforts to reduce it. You won't be able to contend with all the potential causes at once so pick perhaps an individual product line and begin working on that.
Once you begin to lower WIP, watch for other areas that can come up as problems.
An easy reading book that does a good job of illustrating some basics I think is "The Goal" by Eliyahu Goldratt. It is written as a novel rather than a "text book".
Regards and Good Luck,