Maybe I'm partly imagining things, or being selective in my thinking, but even if that accounts for some of it, there is a fact that the book-into-film method is beginning to shift, or has already shifted.
I Am Number Four, a book released in August 2010 about an alien in high school (to simplify the storyline) had its own movie come out in March 2011. Not only did the film makers start work on the adaptation before the book came out, but they did it all at breakneck speed.
Twilight as a franchise has been pumped out every year, or even every six months, and work on the first film couldn't have begun much after the book turned into a hit.
One might list Harry Potter among these, but while I think it is itself part of the shift I think it happened during the tide and managed to escape the mistakes made in other examples.
Why is this happening?
It's quite simple. People's interest and taste changes constantly, more so these days when we have so many new books, movies, music, and electronic gadgets to choose from. Trends in the "what's popular among teenage girls" are in and out at a moment's notice, and if you want to climb on that bandwagon you'd best do it fast.
Someone, whether it be a publisher, author, or affiliate of I Am Number Four let someone else in some film studio know about it, and work got started before it came out. Why? It was a popular book or film formula currently, and expected to be a hit. But not only do books usually take longer to become hits, a book has to become and then stay a hit before it establishes a fanbase that will follow it to the box office.
Twilight is a similar example. The books were a raging success, but for how long? I'm sure people realised that the quality of the story and writing wouldn't allow it to be successful for longer than the fashion stays "in" and if you wanted a piece of that action, you would have to be quick. Which is precisely what went on: before the books were all released, before the blaze could die down, the movies were pumped out lightning-quick because if they produced them in the normal way, their chances of big rakings at the office would drop significantly.
With Twilight, it worked. At least, they made money and lots of it. With I Am Number Four, they got ahead of themselves and it was a complete flop. But when you start with books that follow the current fashion and produce them with such blazing speed not only are you starting with something less than great, but you're rush creating a cinematic version which can only mean one thing: good or bad, whatever you started with will be degraded in your absurd and money-hungry haste.
Even when it does what the movie producers were hoping, and makes box office gold, the films suffer, from the acting to the editing. Though cinema has always had its sections of dedicated filmmakers who make it for the art (a small section, surely), always had its somewhat larger section of those dedicated to making as much money as possible, along with blends of the two mentioned, these days with more and more films being made and people turning to bestsellers for ideas or even possible hits in literature not yet released, it seems a newer and more shameful sort of scratching for money.
However things go, whether we as a society entrench ourselves deeper and deeper into this habit or break it for some other one, the culture's reasons for and ways of making movies will find someway to deprave itself, I have to say, I'd rather it wasn't this one.