Books adapted into movie encourages people to be interested in the series and to pick it up.

BUT

It does not necessary means the buyer are reading it.

I have seen people buying The Game Of Thrones because of the movie and looking for a cover that have Eddard Stark on it coz it looks cool.

And yet I am pretty sure the fellow wont finish the book. Lo and behold, on a separate case I have been able to buy 2nd hand full set of A Song of Ice and Fire boxset because the previous owner bought it on impulse and have no time to read it. Upon further proding, the buyer poked at the first book, n flipped the last book around.

The rest are left alone.

That said, books into movie increases sales but it does not necessarily means increase reading.