Why does a glass rod rubbed with silk not attract the silk which with it was rubbed even when both are oppositely charged? The glass rod acquires positive charge and the silk acquires negative charge. So both should attract each other, right?
My book says, "The charges acquired after rubbing are lost when the charged bodies are brought in contact." 
What does this mean?
 A: Let me discuss the other frequently used demonstration of charge being acquired: rubbing an inflated balloon against (long) hair.
As the hair becomes charged the hairs tends to stand out, as there is electrostatic repulstion between the individual hairs. The hairs are attracted to the balloon. 
We notice that even when the hairs touch the balloon the charge isn't lost. This is explained as follows: the hair and the balloon are both very poor conductors of electricity. This means that any charge that has build up remains at the location where it was build up. The charge will not flow along the length of the hair, the charge will not flow along the surface of the balloon.
The fact that both are poor conductors is part of the explanation why those substances can build up charge in the first place. When materials that are conductive are rubbed together you don't get buildup of charge because any minute buildup of charge just flows right back.

Glass rod and silk
Comparison with the hair and balloon setup:
Hairs have almost no weight, even a very small charge is enough to get the hairs to move a lot.
In glass rod and silk demonstrations what you can get to move is very tiny objects, such as very small pieces of paper. Any object that is heavier than those slivers of paper (or strands of hair) will not visibly move.
There are devices that can build a large electrostatic charge, such as a van de Graaf generator. Such a device builds up charge over time by having a band run around in a loop, acquiring charge at one end and transferring charge to some accumulater at the other end.
In the case of rubbing a glass rod and silk the amount of charge that you build up is not enough to visibly move the silk cloth. The cloth is dense and strongly connected. To see motion you would need to unravel the strands of the cloth in loose fibers.
A: the surface of a glass is very smooth hence the electrons are very loosely attached to its surface . hence when a glass rod is rubbed against the silk cloth , glass rod looses electrons and become positively charged and the silk cloth becomes negatively charged.
now after they have acquired the charges , when they are brought in contact to each other they get neutralized . as the glass rod is positively charged it is in deficiency of electrons and it is waiting for a chance to accept electrons and become neutral once again because a substance tends to be more stable when it is electrically neutral rather than being charged. and similarly silk cloth also wants to give away its electrons since it is negatively charged and it has excess of electrons.
so when they come in contact with each other glass rod will take its electrons back and silk gives away its electrons and both become neutral. now they do not contain any charge with them.
hence they do not attract each other anymore
A: Glass and Silk are in very close positions in the Triboelectric Series Table.
Glass looses a bit more easily  its electrons than the silk but only for a small degree since both materials are on close positions at the upper positive part of the Triboelectric Series Table.
"A material towards the bottom of the series, when touched to a material near the top of the series, will acquire a more negative charge."
Therefore when rubbed together silk becomes relative negative and glass positive.
However, the charge difference is possible too small to observe any visible effect especially when a heavy silk cloth is used. Try with small pieces of silk to observe an attraction.
Of course for weak attraction, the materials should be brought more close together to observe the effect.
For the final part of the question, as already pointed out by other answers here, contact between electrostatic charged electrical conductive materials will cancel any static electricity build up and return the charge to neutral for both materials with physical contact because any excessive negative gained charge by a material  is released back to the relative positively charged material via an electric current.
Static charge between two electrical insulators in physical contact is different. They keep their charge much longer because there almost no current flow.  nevertheless, eventually with time however due to dielectric leakage from one material to the other they loose their excessive charge and become neutral. Charge leakage to the air is also possible.
