I have an old Esperanto textbook called "Esperanto Lernolibro por Popollernejoj, lau rekta metodo" (Esperanto textbook for popular schools, using the direct method) by Arthur Degen.
It uses the direct method of instruction. That is, there are no remarks or explanations in the native language of the students--- in this case, German--- but it is all in Esperanto.
The advantage of this is that the textbook could have been used not only in Germany, but in England, Finnland, Chile, Japan and Nepal. One would only need a teacher who could speak Esperanto. The disadvantage is that a teacher is needed, the direct method isn't very good for self-instruction.
I have begun to put this book into Esperanto sen Fleksio as an exercise. I think it will be a useful reading exercise for those who know a bit of Esperanto.
Degan Lernolibro en Esperanto sen Fleksio - Leciono 1
Degan Lernolibro en Esperanto sen Fleksio - Leciono 2
Degan Lernolibro en Esperanto sen Fleksio - Leciono 3
Degen Lernolibro en Esperanto sen Fleksio - Leciono 4