That’s a great question, I’d say with the rise of Asia so many companies are targeting android and so Java engineers are arguably more in demand than swift engineers, I’ve heard a number of engineers say this and it’s consistent with my experience in the job market. So in my experience I’d say that java will be enough. As far as the best book focused on mobile I’m not exactly sure, because I learned through trial and error on my personal project and primarily through react native documentation - though I had a few years experience prior in python development. I’ve heard people recommend the book android development for dummies which is about 300 pages, but I’ve always believed the best way to learn to program is through trial and error on personal projects + by the end you’ll have something to show an employer, and over time your skills will get refined, especially working in a professional organization I’ve seen my skills increase exponentially