Dumas is a fairly good choice to learn. While many of his turns of phrase and some of his vocabulary are a bit archaic, it's 19th century French that's grammatically correct and elegant but was also written to be easily read by the working class folk rather than by an educated elite. So it's quite accessible (or used to be... Dumas has fallen out of favour for very young readers nowadays. The littérature jeunesse offer is too large, and Dumas is now considered "difficult". He's rather read by 18/25 y.o., or as a summer read for people who can't bear Proust and, I guess, have already read Millenium).

For Tolkien we'd rather use the adjective "archaïsant", ie: using stuffy/archaic prose on purpose for style (which has been done in French translation as well, an "epic" style that isn't very natural to French, as it happens). Another way the translation is described is "style lourd et ronflant". Let's say it's a bit like a plum-pudding to a croissant, as far as French prose goes

The original is vastly superior, the French translation is really a pis-aller, definitely not worth reading for its own sake IMO(I'd pick French classics of high or popular literature over that translation any time)

I totally agree on reading already well-known works to learn (I did no different, and LOTR was one of the earliest books I re read in English on my own, as it happens), though I guess it just wouldn't have occured to me to pick an English translation of a French writer's novel to learn English (reading Monte Cristo in English, for instance), I'd rather have picked an English novel I knew only in translation, counting in part on the discovery of the original work as an extra motivation (for the reverse, especially the LOTR translation maybe, as the translator made some effort to give his translation an English/British feeling.). My main motivation to keep learning was precisely to get rid of those nasty translations, though...

A good challenge (and very good novels, at that) might be Paris-based Canadian writer Nancy Huston. Her books aren't translated the classic way; she writes them in English or French, then rewrites them in the other language. Same book, but not a word for word (or for that matter paragraph for paragraph) translation.

In school they had us read the best known Agatha Christie's as beginners (around 13 y.o. - younger than that we were only in textbooks as far as reading English went), later we had "Of Mice and Men" and "The Old Man and the Sea", and finally Moby Dick. Our only british teacher (most were English canadians) tried to get us to read either Great Expectations or Pride and Prejudice, but she made it a non compulsory activity (those books were not in the ministry's programm) and in the end she had to hmm... lower her great expectarions, I guess.