I think Émile Zola wrote the Rougon-Macquart novel cycle in order, basically, to show off.
I haven’t read everything that Zola wrote, but I have read this amazing novel sequence and it was one of my most enjoyable reading projects ever. To be honest, I was a bit depressed when I finished and I put off reading the last book for several months after I’d finally bought it to delay the inevitable. Seven years, 2o novels, what felt like hundreds of trips to random second hand book shops and now I’d never be able to read a new Rougon-Macquart book ever again.
All 20 novels are set within the 18 year period of the 2nd empire. This may seem like a minor blip in French history, but Zola treats it as a case study for understanding humanity. Within the 20 novel cycle he explored the major industrial and social phenomena of the time. The novels also follow (and prove) Zola’s theories of inheritance and genetics, and, less famously, demonstrate his ability to play with a huge number of literary forms, from journalism to the pastoral and from social criticism to mythical fantasy.
The books can be read in any order. The general recommendation is to pick a theme that interests you (railways, departments stores, alcoholism) and then wait to see if you get hooked. If you don’t you’ll probably have read a great novel and the majority of the books are pretty much stand-alone triumphs in their own right. It is true, there are some less engaging books along the way, but the hit rate is impressive, and this was one of the most enjoyable read projects I’ve every undertaken.
Below is an image of Zola’s family tree, explaining all of the links between the characters. Personally, I find it pretty difficult to decode, but it might work for you and I feel somewhat obliged to share in case it helps…