I finally tracked down a digital copy after seeing the movie 'Selena' and wanting more about her relationship with Chris. The official publisher is HarperCollins, but availability depends on your region. I had to check a few platforms.
In the end, I read it on Google Play Books. It's usually there for purchase or sometimes as a preview through Google Books. You could also try the Apple Books store if you're on an iPhone or iPad.
It's not typically on subscription services like Kindle Unlimited, from what I've seen. You'll likely have to buy it, which I was happy to do. The ebook works fine, though I kind of wish I had the physical version for the photos.
The writing is very personal, like a long letter from Chris. Reading it online was convenient, but it made some of the sadder parts feel even more intimate, somehow.