is good (Patterson and Hennessy).
Actually, make that, my copy was good some years ago and I have no idea how up-to-date with the latest developments the latest edition is. That may not be a concern to you.
I'm sure you can get a mass of free information by searching on computer architecture; trouble is it is likely to be heavier on why the latest Intel chip is faster and overclocks better, rather than fundamentals. OTOH it is more likely to make light reading than a real text book.