In addition to being less expensive, the Weisshaar book is better suited to Hiyokomaru's needs because it is organized as a comprehensive guide.
Only Volume 2 of the three-volume IPCI set gives instrument repair instructions. This volume does have a lot of good stuff in it, but it is a collection of disparate chapters, each written by a different author. It includes eight chapters on "Counterforms, Molds, and Casts," but nothing on making fingerboards, and five different ways to bush peg holes, but nothing on replacing blocks.
The IPCI set is an excellent supplement for advanced restorers who want to progress past the material in the Weisshaar book, but the Weisshaar book is a much better introduction.