Website full of resources about the entire Kirkpatrick model. The "Resources" tab on the website contains several articles, white papers, and webinars about the Kirkpatrick model and how to implement all of the different levels of evaluation.
More in-depth discussion of when and how to conduct a Level 3 Evaluation. The author also discusses some opportunities and pitfalls that can result from performing a Level 3 Evaluation, which should be considered before starting one.