To view this book as The Silmarillion of ASOIAF is both right and wrong. It does tell of what happened before the Game of Thrones as we know it started, but it isn’t exhaustive. A lot of it is merely a collection of the known tales and history within the Seven Kingdoms. The work itself is the first to mention its incomplete nature and often mentions that we may never know the truth about various events.
That doesn’t hamper it in the slightest. The book is a wiki made readable and you’ll have plenty of surprises and puzzle pieces locking in place as you work through it. A lot of it could’ve been dry and useless, even in relation to the ASOIAF books (and to a lesser extent, the series), but it manages to entertain and fill in the gaps with vivid colours.
Fascinating and highly recommended while you wait for Martin himself to finish the Song.