Hello, new guy here, just signed up for photrio a few days ago so this is my first post but I’ve been in to photography since the late 70s on and off since then.
Got a 501 C/M and a few lenses a couple of years ago because I’m retired now and had to scratch that one last itch I had.
I haven’t heard it mentioned yet in this thread, but as the 500 series is a “system” with a lot of various accessories, I found “The Hasselblad Compendium” by Richard Nordin, to be invaluable to describe what components are available, what they do and what are compatible with which cameras. I got the last edition, published in 2012. Maybe this isn’t needed for everyone, but I found it very helpful in understanding the system as a whole.