Way of the Knight Enchanter

If memory serves correctly, the book you are looking for is a scroll that is in the small alcove where you find Dorian in Skyhold.  I also believe the necromancy book is on the very same floor.  You have to use the search function to help you find it.  The rift mage book is with Solas on his table.

Also, if the Knight Enchanter book is not with Dorian as I mentioned, check the floor around Vivianne.  That, I believe, is the only other location that I looked before I found it.  To get to Vivianne, start at the entrance to the main hall of Skyrim, facing toward your throne.  Watch the doors to your right, the first leads to Solas.  The second on leads to a set of stairs that go up.  Follow these stairs to the secnd floor and turn right.  You will be on the second floor balcony that overlooks the main hall of Skyrim.  To your right and you should be looking down on your throne.  If you turn to the left, you will go up the stairs to a fancy decorated area that leads to the outdoor balcony.  Vivianne should be there.  You will see several books and chairs.  Just search around there.

Good luck with Knight Enchanter by the way. By the time I get everything that I need to start on that specilization, my mage has already become an uter nightmare with the base skill trees that are provided.  I never get around to using any of the Knight Enchanter skills.

I think you mean Skyhold, instead of Skyrim 😛 But yeah where Vivienne is supposed to be, there's absolutely nothing, and in Dorian's little corner I only found the necromancy scroll.

Also if you're talking about not having any skill points left to spend on spec skills, there's an amulet you can buy in the undercroft that resets all of your skills.