How big are you planning on making/ selling the books? Size of books matter for resolution. I would pick 2–3 sizes.

Also hard cover/soft cover.

Your illustration files should all be vector based anyway unless someone requests them differently. Standard resolution for print work is 300dpi, but printers will know their process and can give you specs for anything you’re actually printing. (comics would probably be lower for instance)

For ebooks, there are guides for the different platforms as far as how they want your files set up. But in general you’re more worried about file size than resolution there, and vectors will give you best performance as noted above.

I’d grab a copy of Adobe InDesign if you don’t have one already. There’s a free trial and working through a couple of tutorials might give you all you need in terms of answers. It will save you a ton of time on any sort of book setup.

As far as collaborating, some people love email or chat, but I find having a record of what’s been done and what’s up next, that everyone can check, is really helpful. Basically just give everyone the ability to keep their part moving and easily inform the rest of the collaborators. There are a ton of online project management solutions out there, I like Asana but for what you’re describing, something more visual might be better. Basecamp is good, but they’ve “simplified” their pricing to make it hard on small teams.

There’s so many it’s tough to make a single recommendation.

Good luck with it.

Smashwords.com is the best deal I have been able to find for indy e books.
I haven’t used it yet, but I plan to post some shorts there soon.

They have a full, detailed set of instructions to be read by new posters.
I like that writers choose their own prices.
If things go well with the shorts, I have novels waiting in the wings.
Self publishing is scary stuff. I have put it off a long time.
When you decide, and go with a choice, I would sure like to hear about your results.