Dear Mr. Lamprecht,

There is probably nothing that truly qualifies as "memoirs" of N by N; that is, nothing like Patton's or Montgomery's or any number of Wehrmacht generals of a later era.
You are probably thinking of the "Memorial of St. Helena" compiled by Las Cases (you can find a wikiwakipedia entry on it). This multi–volume work is more properly a collection of conversations with N on St. Helena rather than a memoir (even though one version published in NYC in 1890 was indeed titled "Memoirs of the Life, Exile and Conversations of the Emperor Napoleon by the Count de Las Cases"). First published in 1823, it was a huge publishing success and is available in English (doubtless on Google Books). It is very useful but was also a way for N to paint the picture of himself that he wanted History to remember, so caveat lector...

There are, as has been mentioned elsewhere on this site, a number of other persons who recorded conversation with N at St. Helena on varied topics: Bertrand and Gourgaud, for two.

The fine new set of correspondence published by the Foundation Napoléon is called the "Correspondance Générale" and is available on the French version of Amazon. Again, as mentioned before: this edition does not include pieces that N did not personally sign, so it lacks some of the key items that do appear in the 32-volume "Correspondance" published under N III in the latter 1800s.

Hope that helps.