It's been a while since I read the Takeshi Kovacs Altered Carbon novels, so I don't quite remember the details on how the stacks came about if there were any.
What are the stacks made of and who invented them? Where in the books is that explained, if at all?
It is explained in the first book, Altered Carbon, and again in the later two, that stack technology was reverse-engineered by humans (although not stated by who) after similar technology was discovered in Mars, left behind by the Martians.
From descriptions in Altered Carbon, and again in Broken Angels, we know that stacks are about the size of a cigarette butt and fitted to most humans after they are born.
Stacks themselves are referred to as 'altered carbon' in the first book by both Kovachs and Kadmin, although this could be a poetic reference as opposed to an actual description of the technology.
When describing Kadmin, Kovach says:
In an earlier age, he would have been a shaman; here, the centuries of technology had made him more. An electronic demon, a malignant spirit that dwelled in altered carbon and emerged only to possess flesh and wreak havoc (emphasis mine).
Later, Kadmin asks Ortega:
Where is the voice that said altered carbon would free us from the cells of our flesh?
In chapters 8 and 12 of Broken Angels there are several descriptions of stacks as being 'metal':
Most of it was gone, and the final pieces of vertebrae were being eaten away from the tiny metal cylinder of the cortical stack.
Later on Wardani holds the same stack 'between thumb and forefinger', giving us a vague sense of the size of the stack.
Later in Chapter 12 the sound of stacks being poured is described:
As we watched the skip tilted forward and something started to spill over the lip, cascading onto the deck and bouncing up again with a sound like hail stones. Cortical stacks.
The books never explain where the altered carbon technology for a stack comes from. (Source: I just speed-read my way through the trilogy.) We can only assume the technology was originally human-created, since it's unlikely that it came from the Martians:
The Martians, once the scourge of the old Earthbound faiths as knowledge of their million-year-old, prehuman, interstellar civilisation cracked apart the human race's understanding of its place in the scheme of things. And now usurped by the New Revelation as angels; God's first, winged creations, and no sign of anything resembling a cortical stack ever discovered in the few mummified corpses they left us.
Woken Furies, chapter 26
As further evidence of that, the technology for interfacing human brains with Martian technology is very recent:
I'd seen a few like her before — a prototype variant back on Larimer, where the core of the new Martian machine interface industry was boiling into R&D overdrive.
Woken Furies, chapter 3
Note that Kovacs is only 2 years back from Larimer at this point, so this is very recent tech.
A stack is described as approximately the size of a cigarette butt:
It didn't look like much, impact-resistant casing streaked with blood and barely the size of a cigarette butt with the twisted filaments of the microjacks protruding stiffly from one end.
Altered Carbon, chapter 6
