There are two books. Part one Cosmogenesis and Part two Anthropogenesis.
Very difficult to read, maybe be conversant in hindi terms would be best. Or be able to look up unfamiliar terms.
Written entirely by Blavatsky in a trance while she was bed-ridden. The creation myth (from the bible) was much more informative (than the bible's version) , and also explained much more in detail.

I suggest reading this before Isis Unveiled.
This is the copy I have but there are other copies here that you can borrow.