The 2nd edition of "The Design and Implementation of the Anykernel and Rump Kernels" is the definitive technical guide to the core of the Rump Kernel ecosystem. The second edition of the book is unfortunately currently "out of print". Please check back later. Meanwhile, you can download and read the first edition.
If you find the book useful, you are welcome to show your appreciation by donating a sum of your choice to the author via the link above. If you have a hard time choosing a sum, the suggestion is half of what you paid for the previous tech book you bought.