This is impossible for a language

[eggyk]

Things like "-ly" or "-dly" would get huge coverage too in english. "-lijk" in dutch as well. e?

But the challenge is to find a suffix that may occur in hundreds of words in the same book, and is by itself a word that is more common than all of them.

All the best, --stolfi

Something like "den" or "en" in dutch, perhaps. 

Any noun that ends with "d" will become "den" when pluralised. They both work standalone too:

"en" is extremely common word of course, meaning "and"
"den" is an old version of "de" (at least i think?) which is the definite article "the", but also means pine tree

Both would likely be more common than the suffix version if used that way.