What does explain that the sum of voltage of the batteries connected in parallel is same?
Your book likely doesn't mention batteries connected in parallel because it is using an idealized simplification of a "battery" that makes parallel battery connections meaningless.

In the problem above, the "batteries" are represented by ideal voltage sources.  They have an exact output voltage, infinite capacity to source current, and no internal resistance.  

If you put two ideal voltage sources of the same voltage in parallel, they will behave no differently than a single ideal voltage source.  If you put two ideal voltage sources of different voltage in parallel, you will have a circuit with infinite current because the over-simplified ideal voltage source just doesn't represent reality with high enough fidelity to give you an idea of what really happens.

In reality, batteries include series resistance (voltage changes as a function of current) and discharge curves (voltage changes as a function of state of charge).  To first order, putting two batteries of the same voltage in parallel will reduce the voltage drop across the series resistance (because each battery sources only ~half of the current) and double the battery life.  However, those interactions are fairly complex, and I would recommend you ask a more specific follow-on question if you have something specific in mind.