The Little Book, is probably the Book that Daniel was told to seal up.
It contained things that were not to be revealed until the time of the end, not the end of time, but the end of the times of the Gentiles which takes place the last half of Daniel's seventieth week, with The Great Tribulation Period.
If this is true, then the Little Book is a preview of the things that are to come upon Daniel's people in the last half of Daniel's seventieth week. The effect on John's reading of this Little Book seems to confirm it.