The books are exhaustive enough to be used as a replacement for a tutor in a particular subject.
If the material in a particular book covers more than one area, then it could be used for both NWP slots.
Since the creatures covered in the books are all native to Oerth, exobiology is not really the correct term. Each set of books is exhaustive enough that it would take you a week of study (forgoing all else) to learn all the relevant material.
Metallurgy is indeed a separate skill from alchemy. There would be some overlap between the two, but not a great deal. I would say that if you have a skill level of 5 or greater in one skill, you could give yourself a 10% bonus towards a skill check of the other skill.
BTW, it's Oghma.