I found it hard to get a large scale map for touring California and when I found this atlas I realised it gave me what I needed.
In lieu of UK Ordance Survey maps (sadly there is no US equivalent) this atlas serves well. The atlas is divided into 4 sections. Regional Maps: the US, Western US and California - all very small scale showing major highways and interstates.
Recreation Guides : maps of San Diego, LA, San Francisco, etc at 1:862905 scale along with some details on climate, places to see, etc.
Landscape : the bulk of the atlas showing detailed maps covering the whole of the state at a scale of 1:300000. The maps aren't far off Ordnance Survey quality in this section and the scale is fine for driving. You will still want larger scale for more detail when exploring on foot - but printing off online maps can help fill in the missing extra detail for this to supplement the atlas when planning a tour.
The final section, Metro, has maps of major metro areas (LA, San Francisco, etc.) at a scale of 1:157895.
I would still use a Sat Nav (GPS) to help route finding, especially in metro areas, but having a map to get an overview of the whole route is invaluable when touring. The atlas is about A3 size, which isn't a problem using in a car - maybe better than a larger folding map.
So overall I would definitely recommend this atlas as a planning tool and a great companion on the road in California.