I would like to improve my writing style, grammar, and vocabulary, but I'm having a hard time finding a good exemplary book. Most of books I find perfect at first, turn out to be full of flaws and/or informal expressions.
I'm looking for a non-fiction English book that is written in formal -but not obsolete- style. An advanced level book with impeccable vocabulary, grammar, and structures that I can safely follow in my essays (for instance, in the GRE analytical writing essays).
In other words, I'm looking for a "holy book" for professional writers. Please let me know if you know one.