First Edition of P.L.

A bit of Googling turned up the text of the original 1667 edition of Paradise Lost in 10 books, easily readable online here.

Barring several additions and minor modifications, the text is the one we have. But the division into 12 books for the second edition of 1674 (which is now the standard edition) came about by halving two very long books:

The original Book 7 became what we now have as book 7 and book 8.

The original Book 10 became what we know as book 11 and book 12.

Here's a collection of essays that looks at the first edition.