The one book I do find terribly useful when I want to learn a language as soon as possible is a cheap, pocket, travel phrasebook. Start speaking right now – even if you have only learned one or two words and phrases! It will be hard to begin with, but you will progress much quicker. The only solution to this vicious circle is to get out of it entirely.
This logic is faulty as there is always something missing, so you’ll never, ever feel 100% prepared. No more waiting! People are always over-preparing until that day when maybe they’ll be ready to start. When you are ready for it, and very comfortable using the language then it’s a good idea to come back to textbook materials to tidy it up. Embrace this and you will learn quicker and get your point across more efficiently. Grammar does NOT help you to speak a language – it helps you to improve your language skills. If you’re interested in learning how to speak Spanish/French/any other language with human beings, and not just to pass exams or language courses, then put aside the books.