p. 190 only says that the charger will work with the EN-EL9 and EN-EL9a.
p. 189 only shows that the camera battery is the EN-EL9a.
My guess is that the D40 will work with both batteries but the D3000 will only work with the EN-EL9a. Often the batteries with the suffix contain a chip that is used by the camera to help monitor the charge. If he EN-EL9 fits it the battery chamber it may work in the D3000. It's possible that the EN-EL9 will not fit in the camera, don't force it if it doesn't go in easily.
Gary in SE Michigan, USA. Nikonians membership - My most important photographic investment, after the camera. D4, D810, D300 (720nm IR conversion), D90, F6, FM3a (black), FM2n (chrome) YashicaMat 124, Graflex Speed Graphic 4x5 My Nikonians Gallery & Our Chapter Gallery