Computer Architecture: A Quantitative Approach by Hennessy and Patterson: https://www.elsevier.com/books/computer-architecture/henness... is the standard text in this area, though it's more focused on architecture and microarchitecture design rather than the actual circuitry.
I strongly recommend all programmers at least skim Hennessy and Patterson. It will give you a much better understanding of what's happening inside the processor, the cache hierarchy, etc. Not just how it operates, but _why_ it's designed that way and what tradeoffs were involved.
The really advanced stuff is usually kept secret for obvious reasons, but if you're just looking for some OoO/superscalar stuff, Intel published a series of papers about the P6 microarchitecture and there are also plenty of patents (many of which are now expired) to refer to.
The A2I was used in the Blue Gene/Q supercomputer line. A2O is out of order variant which was not used in a product. They have multi-threading, hypervisor features, superscalar, etc. They are "real" cores, not state of the art but probably not too far off.
Not likely to be something you can easily just pick up and learn without good HDL knowledge and working understanding of processor implementations though.
In general, at one time the full Sun Sparc Processor was available to developers (complete with transparent encrypted memory manager unit), but forget about finding the source these days. Also, it is a bit overwhelming for 1 person to cover. =)
