If you are older than 17-18, some formal study of Spanish grammar is almost mandatory. I don't think that just a textbook -it'd be boring, to say the least-. One niece of mine was raised in a Spanish speaking home -she's French- but she got very bad grades in Spanish because she understood but she didn't use subjunctive -or she did the French way- among a lot of things you lose by learning a language in the living room or the kitchen of your home.
The by-product of learning Spanish grammar is that you'd learn a lot of English grammar too (once you start to pay attention to such hidden structures, you process your native language too). By trying to learn some English, I certainly improved my knowledge of Spanish in a greater extent, not because of the "new" things but because many among the old ones "made sense".
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