Additional reference to the number '13' - when talking on the phone toward the end of the movie, 2 of the voices identify themselves as previous "victims" of Room 1408 - "this is Number 5..." and "this is Number 8...." - both of which add up to 13.
Enslin, while looking at the book of the murders, unknowingly, circles the 14th floor and ends up back at the elevator he got off. The classical music that is playing in the elevator was featured in the beginning of "Father's Day", the first segment which was part of the anthology collection Creepshow (1982). The screenplay for that film was written by Stephen King and the segment after "Father's Day" featured, as the lead, none other than Stephen King himself.
When Mike sees him hang himself with bed sheets in the mirror, it's very obvious that the person (a stunt double), is wearing a harness, as the the sheets are centered several inches behind his neck even when he bounces and comes to a stop. It's also apparent that a bungee was used since the person in the mirror bounces back up about 2 feet.
Right after Mike's hand is crushed by the window, the window is closed. Right after the phone rings, and he tries to leave the room (right before the key breaks) the window is open in the background.
What if Mike is still in the room and it was another trick?
A
If the scene had continued, he may have found himself back in 1408 (without Lily, because that would have just been another daydream like when he saw her after he thought he got out the first time).
However, she probably wouldn't have reacted at all and Mike probably would have started freaking out realizing that he wasn't free if this was the case. Also if he WAS still in the room at the end, it would make the ending of the theatrical cut and the ending of the Director's cut pretty much redundant. He dies in the director's cut (sad ending) he lives in the Theatrical (happy ending) to have him still trapped in the room at the end of the theatrical would be just another sad ending therefore might as well have left it as the director's cut. (2012 note: The (or A) broadcast version used the Director's Cut, not only of the end but of several other scenes including a lengthy bit about Mike living in Hollywood.)
Q
Who sent Mike the postcard for the hotel?
A
One Theory It was likely Mr. Olin. Doing exactly what Enslin suspected; Trying to scare Enslin and convince him to stay in another room in order to write about the hotel in his book and give a bit of free promotion for the hotel.
However this plan backfired for Olin when he found that Enslin was not easily convinced and insisted on staying in the room.Another theory It could have been the room itself, somehow knowing that Mr. Olin was preventing anyone from staying in the room long enough to let them be killed. So it sent the post card to Enslin figuring he'd be skeptical enough to not be afraid to stay in the room.
Q
Who is Encyclopedia Brown?
A
Mike Enslin: "Let's 'Encyclopedia Brown' this bitch." Enslin is referring to the fictional boy detective and hero of a series of children's books by Donald J. Sobol.See: Encyclopedia Brown on Wikipedia