The Watchers is a great book - I read it many years ago and I still remember it well.
The original film hacked the book to pieces, changed the main protagonist etc and drastically changed the plot and, in the process, created a film which was simply not very good.
The characters are changed and drastically different along with the plot line which is simplified and has additional villains introduced which effectively change pretty much everything.
This is not really a sequel, more of a remake. It adjust the protagonist back to an ex forces adult rather than a child which matches the book etc (although he is an ex marine rather than ex Delta). The female protagonist is called in as a psychologist rather than being someone whom the dog runs to protect and the owner follows and then befriends.
Also the plot line is heavily simplified and in the process they again actually manage to make this a poor film.
The characters are more similar to the book although the middle of the plot is very different.
This is nothing like the book and does not try to be.
This changes the actual characters to a haunted police officer rather than a haunted ex special force soldier. The female lead is a beautiful scientist rather than a shy retreating young woman. The experiments are changed to be part of a team rather than independant subjects. The NSA are also more villanous than in the book where they are just doing their job. The plot line is natually different as the setup and characters are different and also the book is quite long and detailed and a film will probably have to chop things down.
The closest one to the book is probably Watchers 2 but even that misses it by a mark and is not a very good film :-(