is incredibly useful, not only for vocabulary but for phrases, constructions, and idioms. All searches automatically turn up results both from the dictionary itself and from the forums, so you'll get the threads where previous internautes
have asked "what does this idiom mean?" (And often, for Hugo, they'll be asking it about the exact same phrase you're wondering about.)
Also, the ARTFL project's "Dictionnaires d'autrefois" page
is great for archaic words and obsolete connotations. Silverwhistle can attest to my difficulty with "chaudière," which means heater/boiler in modern French but used to mean "cauldron." D'oh. A quick ARTFL search cleared that right up.
But I really, seriously advise not getting too bogged down in dictionaries when attempting to read in French. By all means, look up key words that are essential to understanding a passage, but don't get so obsessive about it that it's always breaking your reading flow. Especially things like species of birds/plants/parts of a carriage--if you have a general sense of what it is, let it slide. If you're worried that too many things are slipping by you, pick a short passage and translate it so you force yourself to be aware of all the details--don't chain yourself to that kind of attention to detail when you're reading. Unless you're engaged in a nitpicky canon debate (in which case you have a perfect occasion for a translation exercise) it isn't really
essential whether Hugo used the future or the conditional, or whether the connotation of "épouvante" is more "uneasiness" or "sheer shit-in-your-pants horror" as long as you know it's in the general realm of "scary." Reading in French improves your French. Stopping every thirty seconds to look things up takes the fun and fluency out of reading in French. Read fluently, even if you're utterly crap at it and miss things all over the place.
....thus concludes the lecture of the day on "how to teach yourself a foreign language in a way your school will heartily disapprove of." I swear to god that just sitting my butt down and reading all 900 pages of George Sand's Consuelo
for fun--no dictionaries, no tests, no analysis beyond fangirl cackling--was what moved my grasp of the language from "passable if clunky classroom French" to "competent." My only recourse to dictionaries was on words that recurred enough, and that I was clueless enough about, that they bugged me even when I wasn't actively reading, and I looked them up while randomly surfing the internet.
[Dieu] entend ta voix, ô fille des hommes! aussi bien que celle des constellations; car rien n'est petit pour celui devant lequel rien n'est grand.
- George Sand, Les sept cordes de la lyre