A description of how I do the whole debinding thing.

Before we begin:
Yes, there are other methods. For instance, I've tried cutting, which is a lot of work for a so-so result at best.
Microwaving, I don't trust one bit. (Not that I have the right to say there's really anything wrong with it, it just doesn't feel right.)

Okay, so what equipment do you need?

The iron, very important.
The beer, even more important.
The cutting board... Well, you're not actually going to cut anything, are you?
(It's just there because it's hard to make anything with a hard plastic shell stand still on a glass table.)

Don't forget that you will get glue on the iron, so use a second hand or very cheap one.
If you think "I'll just borrow my mom's iron for a bit, she'll never notice", think again and think better this time.

Note that the iron stands at a very ergonomic angle all by itself, very nice.
All you have to do is apply the spine of the book to the iron, and it will start to loosen.
Important Note: The objective here is to loosen the glue, not quite melt it (since that tends to get messy).

First thing is to break the cover away from the first page. (since that's usually glued more firmly)
Tilt the book and heat the front too, it helps with loosening the first page.

Then, start to pull away a couple of pages at a time.

Try to pull, more than bend, the pages away. It doesn't make a difference if you always manage to merely loosen the glue.
But if you do happen to melt the glue, and then fold a page over it, you'll end up with glue spots on the page (which you really don't want).

If you feel that it's starting to get hard to pull them away, bring the book back to the iron and heat a bit again.
Don't wait too long before heating again, some cheap manga have pages that are quite possible to rip.

But! Be careful not to overheat, or it will look like this:

(Yeah, it was on purpose.)
This does not actually destroy anything, mind you. But, if you happen to fold those "whiskers" underneath the page when you're scanning, they will of course disturb the scan.
The simplest way to get rid of the whiskers, if you do get them, is to just drag that edge of the page along the side of the iron a bit, they'll melt and end up on the iron instead.

And what, do you ask, is too hot? The answer is that it depends.
For that book, I set the iron to "wool". It's a good default, but you have to be a little careful about how long you heat the spine.
"wool" is a good default. But as a general rule, for cheaper bindings you need less heat, and for more expensive bindings you need more.
Just take it easy at first, before you've figured it out. (Which shouldn't take too long.)

A normal page looks something like this:

Okay, keep this up for 10-20 minutes, and you get to this stage:

Well, that's all, folks.
I know it might seem like I'm making it out to be too easy, but you have to really try hard to fuck this method up.