The last detailed written material for NAV Service Management was a training document. It was named "50224 Service Management" - for NAV 2009.
Unfortunately the content does not exist anymore on PartnerSource and CustomerSource (whereas other courses still can be found).
The download links would have been: mbs.microsoft.com/.../course50224
mbs.microsoft.com/.../trainingmaterials
I have already sent Microsoft my feedback to add the missing content, so maybe at least us partners will get this material soon.
But there is also an online training available: 80597 Service Management in Microsoft Dynamics NAV 2013 R2 (!)
You can find it here: itacademy.microsoft.com (you will have to use the search function, together with target language English)
You can also use the following direct link to the target Service Module course: https://mbspartner.microsoft.com/NAV/CourseOverview/1050