Why can't a free open source book be equivalent to or better than a more traditional book? First of all the release cycle of the free book should be much faster. Edits and alterations can be incorporated much more rapidly. Giving publishers such control over knowledge can be harmful. Being free does not have to sacrifice quality (this book is written by a mathematician anyway).

Also I recall reading Axler, and I think it covered some more advanced content towards the end. But this book looks equally solid.