I've just been reading Pemberton-Billing's Defence Against the Night Bomber (London: Robert Hale, n.d. [1941]). He talks a lot about slip-wing fighters (and bombers) as a solution to various problems. Shorter take-off distance meant that runways could be shorter (he was worried about bomb damage putting them out of action). For night defence, he wanted an aircraft that was an economical cruiser, so it could stay aloft for a long time; but it also needed to be fast in order to overhaul and intercept the enemy bombers. The extra wing area of the slip-wing would take care of the first requirement (though he also suggests variable-area wings as a possible solution). The fighter underneath would then be dropped when the enemy came along. The difference between P-B's slipwing and the FH.40 is that in P-B's vision, the upper wing of the slip-wing would itself be a piloted aircraft, and would return to base after dropping its partner. Sort of an upside-down Mayo Composite. He does consider jettisonable wings, but considers that his idea has more advantages: controlled descent for the slip-wing, less danger of losing power on take-off (extra engine), more company for the lonely patrol pilot (!) I haven't come across any original drawings of one, but here's a modern rendering.

He also proposes slip-wing bombers and towed fighters. P-B never did lack imagination, that's for sure.

OTOH, what I know about aerodynamics is largely derived from _Neville Duke's Book Of Flying_ which is cool, but far from exhaustive.