This book, by John Butt and Carmen Benjamin, is for people who take their Spanish grammar very seriously. It is a comprehensive reference guide that goes into gory detail on all things grammatical in Spanish. 

To give you a sense of how detailed A New Reference Grammar of Modern Spanish really is, I will provide you with some random section headings:

• Gender of nouns denoting non-living things when they are applied to humans
• Subjunctive in relative clauses after superlative expressions
• Cuál: dialect differences
• Constraints on the passive with ser

I appreciated some words of advice offered in the opening of the book:

Foreign learners should adopt this policy of specializing in one national variety rather than trying to absorb elements from more than one type of Spanish. The latter option is likely to lead to competence in an artificial and sometimes comical type of Spanish full of things as strange as an Englishman calling nappies “diapers” with a British accent or an American calling potato chips “crisps” or standing in line “queuing.”

This point was especially interesting to me in light of my having struggled with a number of Spanish grammars that use vocabulary and pronoun forms from Spain. My Spanish is Latin American, specifically Mexican. The differences among dialects are significant enough that I would love to see more books in the Spanish language-learning market tailored based on region.