I think a platform is very important. There's tonnes of evidence that it causes agents to reject you if you don't have one. Here's a topic on
http://www.creativeburrow.org/index.php?topic=1718.0 which has multiple good stories dropped because of no platform.
I would really hope every author has an author site where they advertise all of their books in a central place....but I would NEVER trade it off for a bunch of book sites. For starters, if you havent published it, it's going to be hard to get viewers, and with no central place to find your books people wont know where to find info on you. Not to mention it's a waste of space when you could buy one domain and hosting per year instead of multiple domains which add up quick.
You can have fan areas for each book/series on your site if you want, either a forum type thing for each one or one forum with categories/sections for each book. I'd prefer the later, but it depends on your fans I guess.
If you look at the way movie studios do it they have websites for each movie. I don't think that's worth it, but I havent looked at stats or anything. It's probably worth it for maybe for 6-12 months while there's hype, if you make it redirect to a page/section on the studio's main site when the hype is gone, but otherwise it's pointless IMO.
So I think if you want a book site you would need to buy the domain for a year, and after 6 months I'd redirect it to your author site unless there's hype (and potentially stop redirects if there's a renewal in hype and you want to). Regardless, you should ALWAYS have a prominent link back to the author site on every page. But you should ALWAYS have an author site, and you should have and build that first!
Hope that answers the question or helps any, I have a splitting headache and most of it is opinion so I could be wrong, I'll research later if I get time
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