Music theory

Although i've always relied on Google for most of my learning, some times, friend recommendations turn out to be great. I've tried many ear training apps on the playstore and also some music theory apps(honestly, the later ones were kinda boring), this app one of my friend recommended beats them all.
Hookthory (no spaces - someone forgot to take their SEO lessons) - This website/app has 2 books each around Rs.1000. The theory is designed to be taught along with the interactive tool they have on their website(you can use just the book if you want though and refer the website later).
Although you might find a pirated copy of this book if you look at the right places, i'd recommend you pay for it if you can afford it.

What you'll learn -

• How is popular music structured.
• Pointers to compose your own music and to recognize patterns in existing music(No kidding). You'll know what it means for a song to be in a particular key.
• The rules that distinguish music from noise.
• The Relative Notion in music - This really does simplifies many things.

You can check the table of contents and other details of the book here or visit the official website.