I already know the alphabet; Korean is known for the easy alphabet and hard as hell grammar. Anyways, thank you! The book you found looks like a good find, positive reviews and cheap compared to other highly rated books I found. Good looking suggested books too. I'm probably going to pool resources for a college book, pirating's never been my thing even though I have no fucking money.
Good chance you already heard of several or all of these, but…
Lingodeer is a bit better app than Duolingo. More structure, teaches words in context and grammar right away.
If you already know a lot of words and grammar u can use Naver dictionary to learn more. Naver has a whole subsection of their website dedicated to foreign speakers and learners.
HiNative is made by the people who made Lang-8, it's an app where you can ask natives questions but again, assumes you know at least some basic shit. Speaking of Lang-8, they stopped new signups but if you just happen to have one from a while ago it could be useful.
Youtube and web Talk To Me in Korean. I haven't used it because I'm lazy and stubborn but they have free courses, native speakers, and free Youtube videos on all sorts of subjects. I heard people say they got very far with them but IDK about quality, sorry. Just throwing things at the wall here.
Or make a new (separate) Twitter account and post in Korean and ask questions while posting about daily life or your interests. I'm only in the super beginning phases of Korean, so I'm not much help beyond this, but I really see this as a good step since it kinda forces you to practice.
Make some Korean friends somehow. Whatever you do, at least try to be genuine with them in terms of friendship. If you just want a language partner I'm sure you could find one by asking around.
I have heard that written and spoken Korean can be very different, so keep that in mind if you didn't know that already.
Don't be afraid to make mistakes. Learn from them. Mistakes will be funny and you'll laugh at them later on down the line. Oh no, that's generic language learning advice…