To be honest... I haven't actually read the book myself... :oops: - hopefully I will get round to reading it one day! There are just tooooooooooooo many books to read in this world - one can never read 'enough'!
All I know is that it is more explantory than the film - as, I presume, most books are...
I know of few books that translated completely and perfectly to the screen (well, no books - it just ain't possible!)...

I presume that the 'visitors' on the spacestation were the people that were most in the thoughts of the crew.

I believe that the planet embodies some sort of 'creation field' where thoughts are transmuted into reality... The visitors came back in their best light... And they kept coming back, because the people they visited still kept thinking about them/longing after them... Until they got far enough away from the field, they would keep coming back...
But then there was that device - some kind of particle accelerator/smasher than annhilated the particles of the physical presence of the visitors... Perhaps it also somehow affected the thought patterns of the crew, so they wouldn't expend enough thought to bring back the visitors again... Because if the field was neutral, then the visitors would just keep on coming back no matter what...

Heck - I think I'm going to find the book now!!! :P