This topic has come up many, Many times in my life, both online and in real-life experiences.
It seems as though many people who read a book before watching a film interpretation of it generally prefer the book to the movie because of the increased depth, time to discuss everything, detail work, etc. etc.
First of all, let me state the thesis of this post:
I believe that books and movies are for different audiences with different needs. Books are not better than movies. Movies are not better than books. Which is better To You is based on what you desire out of them. I am interested on getting a discussion going on this topic, as I thought about a concept while I was at work (I have a fairly mindless job

):
An analogy:
Book -> Movie =
MIDI -> MP3.
In other words,
A book is similar to a movie, just as a MIDI of a song is similar to an MP3 or CD version of that song.
(I know, I know, 'here goes action9000 again with his MIDI junk. MIDI this and MIDI that, when will he stop?' Well, the answer is "never"

: LOL. It's just an example that came to mind when I was at work. I don't intend this to be a MIDI discussion, and I apologize if it turns into one! B) )
(if you just want to get to the point, skip this section)
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What?Allow me to explain this principle that I noticed:
A book contains all of the information needed to interpret the story the way the reader desires. The 'instructions' are all located in the book. The book contains information that can be interpretted differently depending on the reader, such as how a room or character may look, or the tone of voice a character speaks in, or even the distance from one location to another. Everything can be interpretted as the reader desires, with full control over the story experience.
A movie contains a "precompiled" rendering of the story into an easy-to-take-in, "simple pleasures" experience. No strings attached, the film presents its interpretation cleanly and efficiently to the watcher. The data is organized, and everything, right down to the expression on a character's face has been pre-determined. As a result, less interpretation and brainwork is necessary for the watcher, and the easy-access enjoyment is within the grasp of anyone willing to plug the movie into their video player. It's quick, easy entertainment in a small fraction of the time as reading a whole book.
How does this relate to the MIDI/MP3 comment? A MIDI file contains all the information needed to interpret the song the way the listener wants. The 'instructions' are all located within the MIDI file. The file contains information that can be interpretted differently depending on the sound card or MIDI synthesizer that's reading the file. On every sound card, a MIDI file will sound differently. Load a MIDI into a dedicated orchestra synthesizer and it will sound like a live orchestra to anyone but the most critical audiophile. Load it into a techno synthesizer and suddenly it has a completely different sound. The data can be altered freely to fit the needs of the listener. MIDI files may need to be tweaked to suit the synthesizer that it's playing on. MIDIs attempt to be a "one-size-fits-all" technology, sounding good on any synthesizer, but that simply doesn't happen often.
An MP3 contains a "precompiled" rendering of the song into an easy-to-take-in, "simple pleasures" experience. no strings attached, the MP3 presents its interpretation cleanly and efficiently to the listener. The data is organized, and everything, right down to how hard the guitar is played 31 seconds into the song, has been predetermined. As a result, less interpretation and tinkering is necessary for the listener, and the easy-access enjoyment is within the grasp of anyone willing to hit Play on their sound system. It's quick, easy entertainment, often in higher audio quality than MIDI synthesizers on many computers, and can be accessed instantly. Mp3s are designed to sound the exact same on any system they are played on, and they do (minus the speakers they're playing on).
Many people will argue that an MP3 is better than a MIDI file. Many people will also argue that books are better than movies. I consider this simple proof that neither one is better. Both of these technologies use the Exact same set of principles, yet the preferences contradict. It comes down to the needs and wants of the end user, and for what purpose they want the movie or book for.
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My point?
A book is no different from a MIDI file. They serve a very similar purpose: They're flexible, and easy to be interpretted to the needs and wants of anyone who wants to give them the effort to do so. These offer far more flexibility than the alternative, however they are in a more raw form, and require more conscious effort of the reader/user to be fully functional.
A Movie is no different from an MP3/CD. They serve a very similar purpose:
They're pre-compiled entertainment, which can be enjoyed at a much lower cost of time and effort. For many people, these are the media of choice because of their ease of use, high level of entertainment, and easy accessibility. These can be enjoyed with relatively little effort. The downside? There's little room for interpretation (compared to a book/MIDI) and there are few tweaking possibilities for the end user.
As for my personal opinion, I prefer movies to books because I am not willing to devote the time and effort needed to read a book. I enjoy a good movie very much, and I'm more than content with that.
MIDI files are often not high-enough quality for me, and I prefer MP3s to MIDIs, though I use MIDI technology as a starting point for all my work.
Everything has its place. It's just understanding that one is not necessarily better nor worse.