Not at all. The shelves are 18 mm thick multiplex, but they all have a thin 20 mm strip of MDF covering their front edge (overlapping the shelves by 2 mm downwards), so the effective shelve thickness is 20 mm.
The bird's beaks vertically divide each case in increments of 53½ mm. The four practical shelve heights this allows are:
A: 53½ mm × 4 - 20 mm: 194 mm
B: 53½ mm × 5 - 20 mm: 247½ mm
C: 53½ mm × 6 - 20 mm: 301 mm
D: 53½ mm × 7 - 20 mm: 354½ mm
Other configurations don't make much sense, although I do use (53½ mm × 2 - 20 mm = 87 mm) for the special atlas shelves at the bottom.
You may have to adjust these sizes depending on the books you own (particularly paperbacks; they can have regional variations in height). If you go for the bird's beak approach (looks really classy, but a lot more work) you could increase or decrease that basic unit size of 53½ mm.
Test it all in a mock-up before you build it. :)
I used a custom jig and a router to cut the bird's beaks from 18mm multiplex (went through three router bits; gruelling work that), but you can also buy them pre-made.
Feel free to contact me on Twitter if you need more details. I'm only an amateur woodworker though. I really should write up the process of building this beast some day.