Writing is a solitary occupation.

Don't tell me the moon is shining, show me the glint of light on broken glass.

At risk of sounding foully pompous I think that writers' groups are probably very useful at the beginning of a writing career.

And yes, there's a simplicity to writing books because you're not a member of a team, so you make all the decisions yourself instead of deferring to a committee.

Then you start another book and suddenly the galley proofs of the last one come in and you have to wrench your attention away from what you're writing and try to remember what you were thinking when you wrote the previous one.

One book at a time... though I'm usually doing the research for others while I'm writing, but that sort of research is fairly desultory and I like to stick to the book being written - and writing a book concentrates the mind so the research is more productive.