try typing it, I keep pausing every other letter thinking "is this the next letter, it doesn't feel right!"

ssuubbookkeeppeerr

"subbookkeeper" does not appear in the dictionary .

Not in your dictionary

"subbookkerper is the only word in the English language to have 4 double letters in a row "- dictionary.com

subbookkerper

ommergerd, subbookkerper.

I'm not convinced it's actually a word either.

Note that your quote from dictionary.com is from a trivia page, but that actually searching for a definition of "subbookkeeper" doesn't turn up any results. A google search turns up Urban Dictionary as the top result, and subsequent pages seem to either be "community" dictionaries, trivia pages, or language forums where it's status as an actual word is debated.

google returned dictornary.com as top result for me.

Im not a lexicologist but I think I recall these 'words' having special names. Bookkeeper is a word, and you can add prefixes/suffixes to words, if a word has a prefix/suffix. such as Datable and undatable. - http://www.prefixsuffix.com/rootchart.php

I tried googling, but it didn't turn up any result.

also, fun fact.. there is a name for when you add a word inbetween a word. e.g. That's rea-fucking-lly cool dsm
IIRC its Tmesis

but heh, I am prob wrong

-edit-
Lots of google results talk about the specific names for the affix, not the word, so I might be wrong here

@DSM: Wikipedia confirms your memory: "Tmesis" would indeed appear to be the name for that linguistic phenomenon.

As to "subbookkeeper", I suspect that DSM is correct in this: "subbookkeeper" is a valid combination of prefix and noun. Its lack in the dictionary might be just a result of it being (presumably) fairly rare, and thus not considered a full word of its own, but rather a bespoke construct. I imagine that at least some would spell it with a hyphen, as "sub-bookkeeper"; to my eye this looks a little better.