Teun, two of my favorite resources are
A Soil Owner's Manual by Jon Stika and
Soil Biology Primer by Dr. Elaine Ingham.
Jon's book is listed on Permies Book Review Grid, and you can read more about it and where to find it
here.
Elaine's book is available in print at Amazon, but can be downloaded for free (legal download!)
here.
Also, Permies very own Dr. Bryant RedHawk has a fantastic series in our soil forum. There's a link list of his posts
here.