I use the book for Mittelstufe. I find it very useful, a basic knowledge of German helps but it is well laid out.
As an example, one study that stands out for me is Carcassi op60 no 2.
HK starts by showing the difficult sections, then writes it out as block chords before introducing different arpeggio patterns to the piece.
The book groups studies to deal with different technical probleme ie slurs, arpeggios, LH and RH coordination etc.
He also recomends further studies.
A very good book.