Reddit mentions of Modern Spanish Grammar: A Practical Guide (Modern Grammars)

Personally I recommend italki, you can get one-on-one teachers there, some of them for pretty cheap, and even if you only take a few lessons with them, chances are they'll direct you to a certain textbook whose series you could follow if you want. Otherwise you can post in Spanish on there and have native speakers correct your writing, make Spanish friends who want to learn English, etc.

Another useful tool you could use is the 700/150/50 rule I've heard once before. It's "learn 700 nouns, 150 verb conjugations (including what each conjugation means, but going slow with each one), and 50 phrases" will make you good enough to start working on more native material (or at the least, intermediate or advanced Spanish textbooks.)

I grew up using Paso a Paso, but I wouldn't recommend them...

Otherwise you could go the hard way; get a book on the complete Spanish grammar https://www.amazon.com/Modern-Spanish-Grammar-Practical-Grammars/dp/0415273048/ref=sr_1_1?ie=UTF8&qid=1479713428&sr=8-1&keywords=modern+spanish+grammar, learn a bunch of Spanish words on Memrise (I'd say at maybe 2000 you could start approaching more native material, but feel free to constantly go to your dictionary (could be online) and write down the new words you don't know, and at maybe 10,000 or 20,000 you've fully "mastered" the language), and find natives to practice with.

Also there's this website that writes news in Spanish for Spanish learners. A1 is the easiest, then A2, B1 is early intermediate, then B2 is considered "fluent", then C1 and C2 are advanced and professional and "could give a lecture in quantum physics good". These are based off the levels for the DELE test, which are based off the European framework for languages. http://www.practicaespanol.com/

If you want I could PM you the name of the textbooks used at my university up until like semester 5 or 6, but I'm too lazy right now to look them up.