On the pretext that "you don't quite know what you want till you don't quite get it" I'm going to raise this here as it may be impossible/quite a lot of work.
Whilst alpha-testing I decided to try out custom column #pages (integer) as a Groupable item.
With 'Group by first letters' checked the results are:
<<>> [1 #pages, 3 books]
0 [113 #pages, 134 books]
With 'Group by first letters' unchecked the results are:
<<>> [1 #pages, 3 books]
... ...
99 [1 book]
...
nnn [1 book] x a very long list
Unfortunately, neither of which is ideal. What I'd really like for Grouped integer custom columns is something like:
0-99 [n books]
100-149 [n books]
150-199 [n books] etc. etc.
Might such an option be doable in a future release?