If you're planning to go trekking in the Atlas mountains without mule or guide (against lonely planet advice!) this book is essential - especially when taking into account that the only maps available are 1:100000 hand-drawn 'Berber maps' with little and incomplete information.
The first part of the book gives general information about the region in relation to trekking which can be found elsewhere. However, it does feature a few routes more than for example the lonely planet.
Having walked for 6 days in the Mgoun region, we found the route descriptions occasionally very brief (4-5 sentences for a 6h walk) and sometimes inaccurate while other days are described in much more detail. With hindsight the short route description make sense, however, if you're there deciding which way to take you would wish for more information. Fortunately you usually meet the odd shepherd on the way or some kids who are friendly and happy to point in the right direction.
The estimated walk durations are definitely on the optimistic side and walks take easily an hour or two longer even if you don't stop much.
Despite the sometimes short descriptions we were glad to have the book and since there are little alternatives, it is a necessary buy if you're trekking on your own. Note that there aren't any maps in the book suitable for walking; Stanford and Amazon offer maps for the region though.