Heres the deal

Im thinking of buying books on poker to improve my game, i only play texas hold em no limit(with 20 $ buy-in). I got a fairly solid game, at tables (8+ players) im always second or first, sometimes third (but mostly second) . I usually knock out 2 or 3 other players by myself. So what books would help improve the kind of game i play. I think the area i require more help is heads up (+ short stacked). Books on tells would help too i suppose, but i think i got a natural feel with people in general. I heard super system 1 / 2 has some decent sections on tells/mindset and no limit holdem, is this true and are they worth buying both? Please no acronyms for the books...

Thx for your time.