Hello,

> As far as I inderstand the Debian package manager mantains a package
> database. This is the place where it looks for installed packages. So,
> if I have a newer version of guile which is not installed through the
> package manager, the package manager will not know about it.

even if you are locked in a debian stable, you can, and you should in the case of
guile really, install packages from testing, unstable and even experimental [which
is not recommended unless you know what you are doing of course].

So:

	update your /etc/apt/sources.list, add a line with testing source
	update you /etc/preferences
		
		Package: *
		Pin: release a=stable
		Pin-Priority: 800
	
		Package: guile
		Pin: release a=testing
		Pin-Priority: 600

	aptitude update
	aptitude
		search for the guile-2.0 package aptitude entry
		press enter [Menu: Package -> information]
		select and install the testing version [2.0.11+1-1 as of today]

with that setting, aptitude|debian future updates will track and offer you to update
the installed guile-2.0 testing package, while keeping everything else on stable...
there are doc about that, duckduckgo for it...

Cheers,
David