I´m looking for a book on German Literature, describing its history the differents phases and currents, something rather exhaustive and "umfangreich". Is there a classic textbook ? (like for example the 6-volume Lagarde & Michard for the French literature, or a bit like "Der Ewige Brunnen" for German poetry).
The "standard" textbook on German literature is the Kindlers Literatur Lexikon.
This exhaustive encyclopedia comes in 18 volumes and will cover everything.
For covering German literature only there is a much leaner textbook "Deutsche Literaturgeschichte" which in the only recently published 8th edition is quite up to date.