Ask HN: Where can one learn about how computers work?

A friend of mine wants to understand how computers work. I began explaining how binary works and a (very) basic introduction to how the processor, RAM, and graphics card work, but I am looking for a book or other (online?) resource that I can recommend to him (and possibly even read/watch myself) to get a more thorough understanding.

I would appreciate any references, including but not limited to: logic gates, RAM, units of measurement (MHz, MB, etc.), and input/output.

Thank you in advance.