The hardcover books are incomplete, because the collection begins with Franquin's stories - maybe because they're more bankable than Rob-Vell's? Of course, they're great and seminal - most of the time, when a new author or team begins working on the series, they look back at Franquin's stories - but it's a bit frustrating. If you start from them and start asking questions like "How did Spirou and Fantasio meet?", "Why is Spirou dressed like a groom?", you don't have any answers. That said, you never really need to know the answers prior to reading a story. You just get into it and you know from the beginning all you need to know.
There's a spin-off titled "Le Spirou de..." ("The Spirou story by...") where different authors can develop their vision of Spirou in a self-contained, out-of-continuity story. You even get a sci-fi Spirou and a reimagining of Spirou's early years as a groom before World War II by Emile Bravo (a story that spun a sequel in three books).
On a personal note "Spirou in New York" was one of my favourite comicbooks when I got it back in 1994. That was during a special promotion by Dupuis, with special reprints (99% identical to current prints) for 10 francs during a week. Nowadays, there are annual operations with special reprints for 2
