The Bookcases were about 350mm to wide for the space I was filling so I cut them down to size. I also wanted one of the units to be suitable for holding CDs so I added some extra shelves on one of the bookcases using leftovers from the other two.
I am not going to show all the assembly steps. The key thing here is shortening the thick outside frame of the bookcase.
Basically I had this
Step 1: Goal
And I wanted shelves halfway each section as well
I assembled the thing first to mark where I wanted to cut.
Step 2: Cut off end
Step 3: Inner Frame
Step 4: Inner Frame double thickness
Step 5: Inside
Step 6: End Sections
Step 7: End Section 2
Then you need to cut this down a bit because it is too wide to fit back into the hole (if you cut it off at the right place). Cut it shorter than the gap it is going to fit back into, that way you get a bit of adjustment when you fit it all together and things don't quite line up.
Also make sure you take all that cardboard off that is stuck to it or it won't fit back in nicely. I used a belt sander to do that.