Hi Daisy,
Thank you

The above technique of chord substitution is usually referred to as "substitution by function". It's a great songwriting tool to keep in mind.
There's a book by Jai Josefs called "Writing Music For Hit Songs" that approaches writing music and harmony from the perspective of Tonic, Subdominant and Dominant. I found it a very eye-opening book and highly recommend it. I'm not sure about this, but I don't think that the book is printed any longer; I just had a look and Amazon, though, and they have some for sale.
This book completely revolutionized the way that I look at music. I have re-read it a couple of times since buying it. If you have knowledge of music, I strongly encourage to try and get hold of a copy. You won't be disappointed.
Regards,
Noel