Finished bookcase:
https://twitter.com/jdhoek/status/885499829052485632
The four common shelve-heights I chose are 4, 5, 6, or 7 bird's beak 'units' spaced apart vertically.
You can see the configurable parts here:
https://twitter.com/jdhoek/status/881243775288389632
And the second photo here:
https://twitter.com/jdhoek/status/876126132734656514
So beaks at the front and back, with trapezoidal blocks that fit in between. The shelves rest on those blocks.
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
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.