Whatever other guides are out there for Spain or any other country, I still feel the need to have a Michelin Green Guide along for more comprehensive background information on routes/itineraries, architecture, customs, language and history. Although, the English version of the Green Guide to Spain has been slimmed down a bit over the years, it still supplies a lot of information about small towns, national parks and other places of interest that most other guides don't think to include. It's good also for detailed historical and cultural information. Another signature feature that it still provides and that you can't get elsewhere is the "at-a-glance" map reference (inside the back cover) that ranks the principal sights of Spain by level of interest. You can argue the rankings, but it is generally a very reliable tool in planning in-country travel.
Michelin does not include a lot of photo illustration, but it's great on city maps for smaller, regional centers. This has been very helpful to us over the years as there aren't a lot of other sources for this kind of information.
Overall, if you're going to drive in Spain (or elsewhere), the Michelin Green Guide is a good companion to take along. It's valuable even if your travel is done by train or bus. And don't forget the Michelin Red Guide which is still the best for hotels and restaurants.