The Einführung is meant as a comprehensive introduction to as many topics possible in the world of LaTeX, and can be viewed as a roadmap, whereas the other titles delve much deeper into a specific theme, offering alternative solutions, more background information and more case-studies and troubleshoots. The introduction contains only a portion of the other books, whereas the other books always contain everything that has been said in the introduction.
Maybe it is of some further interest to say something about the structure of the series, at least how I see it, and share some user-experience from the viewpoint of a noob.
One can also add to the list an introduction to LuaLaTeX, the comprehensive manual of PStricks and a LaTeX-Reference (all by Herbert Voß), the fantastic printed version of the KOMA-Script-guide by Markus Kohm, "TeX by Topic" by Victor Eijkhout, and a book on METAPOST-graphics by Walter Entenmann (appeared just recently; I don't know it though, as I rely on PSTricks).
In the case of bibliographies it has been helpful for me to first consult the Einführung, and whenever the explanations provided weren´t enough, to then consult the specific book on bibliographies and the internet. In the case of Tabellen I used the specific book directly, as I already had some very specific questions and was interested and not forced as in the case of bibliographies. The same approach has been useful for me with PSTricks.
The series is somewhat growing and the different titles are being constantly updated. They have proven very valuable for someone like me, who is relatively new to LaTeX and seeks help, and who is often confused by conflicting approaches, coming from different times and different people.
