Perhaps there is a BE/AE difference at work here?
In the US, we do call Lehrwerke, as described in #20, textbooks, as in "My principal has given approval for me to get a new textbook for German 3." It is assumed that when I say textbook, I mean more than just a hard copy of a book. Presumably there is a textbook, a workbook, materials on CD-ROM or online. If I worked in a different school with better access to technology, this might all be online, with no actual paper for students to hold in their hands.
When talking about teaching materials for multiple levels, you usually refer to a textbook series. In the US these are generally geared to courses divided by years (first year, second year, etc.), while in Europe I am familiar with materials divided by level (A1, A2, B1, etc.).
Here's an example in English from Klett's website
"The authentic beginning German textbook series for high school is now available both as print and interactive online edition with embedded media."
To me this whole topic seems a bit like talking about phones. We say "dial a number" and "hang up," even though the actions now rarely involve putting your finger in a dial or placing a receiver on a hook. "Textbooks" are often no longer books, but we still call them books.
