Teach Yourself Spanish Grammar (Teach Yourself Series) Book

Very interesting way for any beginning Spanish learners. You are pulled into a story where you apply what you learn at the very end to finish "a mission." I wouldn't recommend it for intermediate or advanced Spanish learners (it's slow and boring), but this would be good for someone who knows only a handful of Spanish or has taken Spanish level 1 and/or 2. It goes at a challenging pace for beginners and uses a lot of useful phrases and unique vocabulary.

One thing I'd keep in mind is that they don't really teach you about the differences between formal and informal uses of Spanish, such as the difference of tú and usted, until near the end. Make sure you research these differences so it can give you a heads up as you're learning phrases. ¡Buena suerte!