I came across this site about vehicles that were made using the platform of the Mini Cooper; the original one and not the newer one. It is a blog called Maximum Mini. The owner of the blog posts regularly. It makes for interesting read.
I was looking for companies or people who were had or were making cars based on the Mini Cooper. I have read of kit cars and plans for cars that are based on existing vehicles (for example, there are kit cars to change the Pontiac Fiero to look like other vehicles).
I first came across the book - Maximum Mini - Amazon. It was listed as an e-book and as printed books. The Amazon site had the price that seems rather exorbitant for a book (at the time that I searched for it, the price was a couple hundred dollars - IIRC - but not it is out of print). I was able to download it from Google Play and read it using their book app. The e-book version was way more affordable.