Dumas Davy de la Pailleterie, born in 1802, generally went by Alexandre Dumas, Dumas being the maiden name of his Haitian grandmother; his father, who did not get along with his own father, had often used it as well. Dumas worked as a scribe for the Duc d'Orleans, who would later become King Louis-Philippe, and while doing that began to write extensively in a wide variety of genres for extra money. He first became famous for his plays. It was his serialized novels more than anything, however, that solidified his name.
The fortnightly book is The Three Musketeers, the first book in his D'Artagnan Romances, originally serialized in 1844. I once read them all, but it has been quite a few years since I last picked this tale of the Gascon and his three friends in the King's Musketeers, from the 'days of less freedom but more independence'. D'Artagnan himself is actual Dumas's free adaptation of a previous d'Artagnan, that of Gatien de Courtilz de Sandras's Mémoires de M. d'Artagnan, who was himself fictionalizing the real-life Charles de Batz de Castelmore d'Artagnan. The real d'Artagnan provides the most basic features of the d'Artagnan story -- he became captain of the Musketeers of the Guard under Louis XIV and died at the Siege of Maastricht -- but Dumas does not really draw on the real d'Artagnan's life. What seemed to have struck him was instead was Courtilz de Sandras's very heavily fictionalized story of d'Artagnan's early life, on which he then built.
A point that does not seem widely to be known is that Dumas co-wrote a lot of his works, and several of Dumas's most famous works were collaborations of this sort with Auguste Maquet, including The Three Musketeers itself. Dumas did the actual detail-work; Maquet functioned as a plot-designer, researcher, and secretary. This approach seems to have been developed by Dumas in the course of writing plays, since it was standard to pass scripts around for modification. In any case, it was because of the fame he developed in theater-work that Dumas became the big-name draw, and publishers did not want to dilute that with a less-known name. By an agreement Maquet's name was left off the title page, but he was paid very well in exchange. Being also a less profligate man than Dumas himself, Maquet died quite wealthy and Dumas poor and heavily in debt, so one can judge for oneself whether it was a good deal. Dumas's own contribution was not slight; he was not merely touching up another author's work but writing the actual vivid, engaging scenes on the basis of someone else's sketch of a story, and there seems good reason to think that he did this with quite a free hand. Some of his contemporaries accused him of running a novel factory and industrializing literature; but no one, I think, can deny that his own literary talents made a significant contribution to the result.
The edition I am using is the complete and unabridged Bantam Classics edition, translated by Lowell Bair.