Starting at the beginning with The Two Dianas ....
Of Dumas' Romances, I have read the D'artagnan Romances twice and last year I completed all of the Marie Antoinette Romances. Now I am going back further in time to read the Romances of Henry II, the first two volumes of which are the Two Dianas, which is really the story of Gabriel Montgommery who accidentally killed the King of France in a jousting accident.
These first two volumes number about a thousand pages, but the story moves rapidly. The two Dianas refers to Diane de Poitoirs, the mistress of Henry II, and Diane de Castro, Diane de Poitoirs daughter purportedly by Henry II. There are three volumes after these two so I'll be reading the Romances of Henry II for sometime yet.
All in all, Dumas wrote 48 volumes of his Romances, not counting his Celebrated Crimes series or his travel books. For the complete Dumas' Romances, see my post entitled 'Attention Dumas' Fans' where I have given all the Romance titles as in the English translations of circa 1890 and published by Little, Brown.