Edith's writings are great, I'd certainly get her book. She also has a
website.
The best book I know of at the moment for pistes is Chris Scott's recently published
Morocco Overland, though this concentrates on the south of the country and he doesn't cover anything in the Rif or Middle Atlas. If you can read French then Jacques Gandini has a series of very detailed
guide books.
However... detailed descriptions with the degree of difficulty is a problem.
When I travel on pistes I save the tracklogs with a colouring system according to difficulty--
green is absolute novice on any bike,
blue is fairly easy with no technical skills needed,
red needs offroad training and
black shouldn't be done alone or with luggage.
Fine so far, but the problem is that these categories are subjective, only apply to the day I did them and in the direction I took--going up steep sections is normally easier than going down. Pistes change over time according to weather and the amount of traffic--the more local traffic, the better the repairs. The best example of this is the link piste between the Todra and Dades gorges.
The first time I did this piste in 2006 I was solo riding with luggage and it took two or three hours, see post #16 on
AdvRider report. When I rode it again in 2008 without luggage in a small group it took 17 hours, the last six of which we rode/crawled in moonlight. There had been bad weather, traffic on the piste had been restricted by erosion and the piste hadn't been groomed since the flooding.
The other thing is that pistes are difficult to categorise. The surface totally depends upon the underlying geology and a single piste might change from beaten earth (great) to small gravel (fine), hard rocks and steps (more difficult) to big stones (hard) or fine sand.
And the wonderful beaten earth when dry turns into a nightmare when wet and it's been churned by traffic. Then, when it's been snowing, locals throw down branches on the track to provide traction...
Anyway, have a look at the various threads on the Morocco forum at
UKGSer. One of these has some 'hidden routes' I've described in Horizons Unlimited presentations.
Tim