Right, having spent the past few days at BA-MA, I can shed some light on this.
There are printed guides in paperback form relating to some subjects
Ther are the large leather-bound guides dealing with most material in the 2 and 3 Reich - these are only available in the reading room.
There are also several leveral arch files of guides for some subjects - again these are only available in the reading room.
However, there is also a new computer system installed in the reading room (Invenio) which is a database of most holdings. It's fast, easy to use, you can order directly from it (soooo much easier than old, arcane ordering system) and get an idea of what to expect. Crucially, it lists the MSg and N files which were often so hard to find guides for in the past. As far as I know, it's only available in the reading room presently; I found a lot of material on it which I would not have otherwise have known about, so it's definitely a big step forward.