> I just found out that Cisco is now using SourceFire/Firepower for these, and is probably going to discontinue the old system.
The legacy IPS is already announced for EOS/EOL.
> 1. How radically different is the new IPS/IDS system? Is it still based on signatures, threat ratings, etc.?
It's still mainly a signature-based system, more or less same as before. Expect an easier tuning and better defaults then before.
> 2. Where can I go to find documentation on this? Any books? PDFs?
Not that easy, Beside the infos on the cisco website the are also trainings like the SASAA 1.2 that start to integrate FirePower. But there it's only one topic of many.
> 3. How long has this been out? Has it been real-world tested?
As an IPS it probably deserves the status "real-worls tested". As a cisco-integrated system, well, I would say it's on the way.
> 4. can I manage these IPS systems with IME, or do I need new software? What about ASDM?
no IME any more! You use the FireSight Management-Center (appliance or VM). I heard that ASDM-integration is planned, but I wouldn't expect that anytime soon.