Balzac is brilliant for a lot of the basic period stuff. The Marriage Contract gives a whole new perspective on Valjean's actions/motivations in the run-up to Cosette's wedding. Plus Lost Illusions is also useful on "young educated people being poor in Paris" - Rastignac's buddy who is in med school also turns up there. Again, great on some weird details, not on others. There's some art patronage politics in Cousine Bette.
I also use Eugene Sue's Les Mystères de Paris (admittedly in translation) as if it were an encyclopedia, but that's usually for a working class source. Costs of things, forms of address, food - Sue covers it all. (he has a grisette go through her weekly or monthly budget in detail after saying how much it cost to set up her apartment. Freaking gold mine, that novel. Pick a searchable copy off Google Books and go to town.)
And I just wanted to add that I love you for caring
I do Yuletide every year, and there are some things I don't sign up for because I realise I will not have enough time to do the appropriate background research in the period or locale within the deadline. I'm "do it right" person, which is how I keep ending up on research hiatus, but it really does make a better product because the research helps the writing - it gives directions one can go and does some of the lifting on its own from time to time.
Have you been pointed to my timeline
? May also be helpful as you go along since you're working on backstory.
What kind of literature and what kind of life is the same question. - Tom Stoppard