Wikibooks is a great idea. Like Wikipedia, Wikibooks consists of content that is public domain. That means that anything there is reusable in any fashion, such as for teaching.
This particular wikibook was the Collaboration of the Month in 2005.
The question arose early in the development of this textbook as to precisely who would be the target audience. Although intended to be a "beginning" textbook on German, many felt that the early lessons were too difficult for younger students with very limited or no experience with German and, perhaps more importantly, limited skills in English grammar. For this reason a textbook on three levels was conceived.
Beginning German (Level I) puts more emphasis on building vocabulary around subject matter interesting and useful to young students.
Basic German (Level II) emphasises grammar, and assumes a greater knowledge of English grammar more typical of an older high school or a college student. If you are just beginning to learn German or attempting to teach yourself, you may wish to try both approaches and see which works better for you, since some people require a strong structural approach to learning a new language while others find this "structure" only impedes progress by adding another layer of complexity.
Intermediate German (Level III), which requires even more knowledge of English, is for college students, preferably for sophomores or juniors.
With even more complex lessons, grammar and vocabulary comes Advanced German (Level IV), which with the most complex and difficult parts of the German language, is for late college students (Seniors) and college graduates.
The last level, which is a review level, but also has cultural facts and the history of the German language, is Reviewed German. (Level V).
An existing, separate text, German/Grammar, may eventually be merged into the lesson modules or developed into useful appendices as a grammar reference. At present, however, German Grammar is an expanding, significant contribution to the textbook; it provides an important reference on German language grammar rules useful to the student working through any of the three levels.