To read the whole 3 musketeers canon, this must be included, as it is volume 4 of the five. It covers the bit just before the Man in the Iron Mask, and reading this certainly makes TMITIM alot more understandable. Reading the whole lot also tells you that none of the films of TTM of TMITIM have done the stories justice.
They are a retelling of the history of the French court over the period from the about midpoint of Louis XIII's reign (TTM) to the early 1660s, when Louis XIV was taking power from Mazarin. The stories are, like all Dumas's histories, heavily romanticised, historical characters being freely - loosely even -intermingled with fictional and semi-fictional (e.g d'Artagnan), and C17 activities being rewritten for C19 sensitivities - King and mistresses never actually seem to bonk, for example, passionate kissing symbolising it all unspoken.
This translation, with copious historical notes, is a good rollicking read, and highly recommended.