Recently, I conducted a Spanish wine seminar at the wine shop I work at. I used this text and Radford's The New Spain for reference purposes. Both were excellent and irreplaceable.
Radford's book is great for its photography and chapters on each wine area, focusing on history, culture, viticulture, vinification and grape varieties; Jeffs' book offers more nuts and bolts. He organizes the main regions into Catalonia, Aragon, Rioja, The Centre-North, Castilla y Leon, The North, The Levant, the Centre, Extremadura, Andalucia, and The Islands. Within each of the main regions, he looks at the various sub-regions and further breaks it down by detailing average rainfall, altitude at which the vineyards are grown, soil, climate, grape varieties (from major to minor) etc...
If you read Radford's book, you get a broader, more polished picture with history and gorgeous photographs. He too divides Spain into the main regions and then looks at each area in context to the region and history. Radford includes information on viticulture but his focus is on making the area more comprehensive in terms of illustration and overview. Jeffs, without the use of photography (there are plenty of maps though), provides the details that bog down narratives. Radford you can read cover to cover as if exploring each area one-by-one. Jeffs is the book you look for in need of reference. It is more like an almanac. He too provides history and a bit more detail when discussing the bodegas. In Radford you get a few notes on the bodegas in Spain. In Jeffs, he provides more information and more detail.
My one gripe with Jeffs is that when you want to look for a specific DO in the book, like Terra Alta for example, there is no actual page number on the table of contents referring you to the DO. You basically have to go to Catalonia and then search page by page until you find it. Either that or go to the index to find the page numbers which is equally inconvenient. There are also some other typos and little mistakes. The North is listed on the Contents page as being on page 112 when really it is page 197. Small, just little bumpy hassles that need mending. Otherwise this book is full of enough information to make this reference book a must have for wine and Spanish wine lovers.