First, only the dielectric between the plates matters. Even if one is immersed in a dielectric liquid, if it does not cover any of the volume between the plates, it does not matter.
Next, the text should mention this, but the second plate must be grounded, to the same 'ground' as the negative terminal of your battery for only then would the two act as a capacitor (an ungrounded plate is of little/no significance in a DC circuit)
Being grounded allows the capacitor to 'pull' opposite charges from the ground (actually, it is a rearrangement of charges on the whole). The same is true if it were connected to the opposite terminal on the battery directly (whichever terminal you assume to be at 0 voltage, that one is actually grounded), but in this case you can probably already tell how the charge flows through battery.
I should have answered your question by now, but in extension, If the first plate was connected to the positive terminal of the battery, and the second plate were not grounded, negative charges would accumulate on the side of the second plate facing the battery connection, and positive charges on the other side (in total, charges are same, just a rearrangement)