You can “borrow” it from the “Open Library”. Check out the ebooks at the top of the list. There might be a waiting list. As of writing this, there are 6 copies of it there and a waiting list of 2. Silmarillion, eBook - search. There isn’t any legal way to read it online except through borrowing or buying it because it is still under copyright.

It doesn’t seem to be available on kindle on amazon . com for some reason but is available on the uk version of the site here: J. R. R. Tolkien: Amazon.co.uk: Kindle Store

There you can read the introductory matter including part of a letter from Tolkien. Or buy it. You can read kindle books online or on your PC and don’t need a kindle device.