I somehow put the vanilla Lever into a GT Bookshelf, which seems to succeed (the Lever item is gone and the “Book” appears in the Bookshelf. But I couldn’t remove it by right-clicking the slot.

putting a vanilla lever or button into the shelf turns it into a hidden switch in the shape of a book, where clicking it will make the shelf emit a redstone signal. This is a feature, but has the side effect of you having to use a hopper to remove the thing, or to just pick up the entire shelf.