Reddit mentions of Digital Design and Computer Architecture

C targets a virtual memory system and instruction set architecture (ISA). It's an abstraction over the hardware implementation of the ISA. Those worlds are just different, and you'll gain a better understanding if you just study them separately.

for computer architecture, I've found two books to be most helpful.

https://www.amazon.com/Digital-Design-Computer-Architecture-Harris-ebook/dp/B00HEHG7W2

https://www.amazon.com/Structured-Computer-Organization-Andrew-Tanenbaum/dp/0132916525/ref=sr_1_1?ie=UTF8&qid=1536687062&sr=8-1&keywords=tanenbaum+computer+architecture&dpID=41B7uYANs%252BL&preST=_SX218_BO1,204,203,200_QL40_&dpSrc=srch

there is a low level operating system book that uses a lot of C code to explain how to build a kernel. This might interest you

https://www.amazon.com/Operating-System-Design-Approach-Second/dp/1498712436

Don't buy an FPGA board yet if you're just getting started and don't want to spend too much money. Most of the development and simulation will be done using the IDE provided by the FPGA sellers (e.g. Xilinx Vivado, Altera Quartus Lite, etc).

Buy a good logic design book and just start learning the concepts of logic design and computer architecture. I don't know what new book is popular now, but I used Digital Design and Computer Architecture back in college. You can buy an older edition since the concepts don't change.

For actual VHDL, you can find tons of tutorials online. Just keep in mind that writing an HDL is not the same as programming. In programming you give instructions to a CPU to do all kinds of stuff. In an HDL you are physically placing circuits in a FPGA/ASIC so it won't behave like a program.

If I had to recommend a basic intro book, it'd be the following:
https://www.amazon.com/Digital-Design-Computer-Architecture-Harris-ebook/dp/B00HEHG7W2/

The new edition uses ARM. I haven't run through this book to say if it is good yet.
https://www.amazon.com/Digital-Design-Computer-Architecture-ARM-ebook/dp/B00XHN8RI4/

For the undergrad CS computer architecture class, we used the same H&P book and the SPIM simulator. For the graduate course, the Computer Architecture: A Quantitative Approach and Simple Scalar were used.
http://www.simplescalar.com/

For EE courses, I mainly remember using at least 3 different books by Mazidi, Triebel, and Brey .

www.amazon.com/8088-8086-Microprocessors-Programming-Applications/dp/0130930814/

http://members.ee.net/brey/tech.htm

http://www.microdigitaled.com/

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There are plenty of good online resources on computer architecture.
Examples for Cache
https://en.wikipedia.org/wiki/CPU_cache
https://en.wikipedia.org/wiki/Cache_algorithms
http://www.ecs.umass.edu/ece/koren/architecture/Cache/tutorial.html
http://www.pcguide.com/ref/mbsys/cache/funcComparison-c.html