Edgar Rice Burroughs began publishing work almost 100 years ago, Understandably in time much has changed and there have been many derivative works based on his original masterpieces. Below are instructions explaining how each functions in conjunction with each other.
Canon
Canon is defined as any work written by Edgar Rice Burroughs that can be connected (however loosely) to his martian tales! These works include his Pellucidar, Tarzan, Carson of venus, the moon maide series, and the two books The Eternal lover and The Mad King.
Artwork
In regards to art, the art must be accurate to the original work that is the only rule. Generally if artwork drawn by Edgar Rice Burroughs son John Coleman Burroughs can be found it is preferred, as he co-wrote one of the books in the martian tales.
Other Works Considered Canon?
Additionally works approved by Edgar Rice Burroughs estate could be considered canon. The upcoming series by marvel John Carter is said to be canon, thus it will be this website policy to consider an non-contradictory material from the series as canon.
Majority Rules
Sometimes there are conflicts in canon within the original works, generally speaking newer material is considered more canon. But, if there is a vast majority of sources saying otherwise then the newer material could be treated as a continuity error!
Non-Canon
Any work that is not written by Edgar Rice Burroughs, or not approved by his estate. It is very simple to understand!
Other Continuities
Somtimes series take place within their own self contained continuities independent from Edgar Rice Burroughs original works. These series are treated as their own self contained entities, treating the work and original content to the series as canon, but only to itself. To all other works these are considered non-canon