I think there are probably many of us that are interested in this side of the "enterprise" but SAP is sort of its own universe and difficult to understand/penetrate from the outside.
Most of their detailed documentation is only available for people who take up their courses (yes, they are that evil) and certification exams.
I do remember quite a bit of it was available on rapidshare though unless you were already working on something specific you wouldn't know what exactly to search for.
To start with, I would suggesting getting a book on SAP R/3. More specifically the SAP R/3 handbook by Jose Antonio Hernandez. I don't know of any thing available for free (legally) that is as good a place to start.
As for the product offerings from SAP, they more or less seem to be components that sit on the same application stack. So getting into one of these applications could provide insight into what everything else does.
Disclaimer: I work as an SAP Basis consultant (that's sort of an application admin + OS admin + DBA)
Or take a really expensive class from SAP (usually paid for by your employer).
The truth is that (it seems as though) SAP doesn't want people to be familiar with the system. Their distribution channel is consultants, and consultants make more money if the system is opaque and hard to get up to speed on.
This will be the downfall of SAP as we know it IMO (and Oracle, and every other enterprise application that relies on a technical consultant's input), the deliberate opacity of the system for the sake of the distribution channel.
Proprietary lock-in (or dial up the "exclusivity" meter on knowledge and any budding subject matter experts). Enroll some of the customer personnel in expensive training only available from SAP (or SAP "approved" vendors).
Contrast to open source models where anybody can become fluent with the code or platform and can be hired or contracted to work at a fraction of the rate a SAP (or to be fair, any proprietary, lock-in "turnkey" "solution").
While I departed the corporate IT world a few years, I have seen the same shenanigans with the likes of Oracle, SAP and others too remote to be recognized by many here. In fact, I owned a company that profited as a "service partner" for a data communications legacy "commerce" software suite maker.
You won't get a SAP job that way, but it might give you enough of an edge to take on SAP-related projects (think "sure, I've done webservice calls into Salesforce and I know how that works with SAP XI, happy to work on your integration project").