I used this guide while completing a full traverse of the GR131 through all the islands in winter 2011/12. I used Cicerone guides exclusively and my comments relate to all Cicerone guides to the islands. (Gran Canaria and Lanzarote were not waymarked at the time of my walk).
First, these guides are up to Cicerones usual high quality. The route descriptions are excellent and you are unlikely to get lost even without a map. The info on accomodation and buses is very useful. Ferry info is necessarily vague as the timetables change regularly.
The guides are obviously aimed at day walkers using a fixed base for their walks and as such there is some important info missing for long distance hikers on the GR131, notably water supply points. I often found water at ermitas and picnic places (particularly in the western islands) although the text didn't always mention this. Consequently I often carried much more water than I needed. In the eastern islands water was mostly scrounged from cafes and bars and it was normal for me to carry up to 5 litres.
As wild camping is illegal the guides don't give any suggestions for night halts under canvas. On the forested islands it's easy to find somewhere discreet for the night, on Fuerteventura and Lanzarote much less so.
Trekking the GR131 in a "oner" is possible but it's pretty tough mainly because of the lack of water and legal campsites.
Probably the best guides available in English.