What are /r/eebooks' favorite Products & Services?

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Maybe Perzold, Code: The Hidden Language of Computer Hardware and Software, or Nisan & Schicken, The Elements of Computing Systems: Building a Modern Computer from First Principles?

Haven't actually read them, but those two are quite often recommended.

When I was first starting my BTEC, I found the John Bird books helpful.

I'd recommend Computer Systems: A Programmer's Perspective (currently reading it) and the course based on the book.

Doesn't seem like there is an electronic version of this book, but you can buy a hardcopy from Pearson or Amazon.

That book is not available for free. You can get it from Amazon though.