In one of the Turkish books I have, the word oturun is included in a dialogue. It is in the context of a conversation in a carpet shop but I cannot find an explanation of the word :-S

I assume it is otur-un - sit!
But the imperative should be otursun or oturunuz......is oturun a shortened version of oturunuz or is it something else?

Your assumption is correct. Oturun is the imperative of plurial "you" but maybe in english it's better to to translate as "please sit" if it's said to a singular person.
Otur for singuliar, oturun for plurial, oturunuz is also for plurial but it's rarely used, it sounds very very formal.
Always the same construction, you use -ın, -in, -un or -ün suffix. yapın, bakın, gidin, okuyun, gülün, ağlayın...

Is the imperative "otursun" not used?

This is my version

Imperative
Sing Plr
gelmek - gel - gelin - come
gitmek - git - gidin - go
görmek - gör - görün - see
okumak - oku - okuyun - read
yazmak - yaz - yazin - write
bak - bak - bakin - look

It is same with oturmak

oturmak - otur - oturun - sit down

Am I right?

When you want to be formal you use -iniz, -ünüz, -yunuz.
Just add to the verbs used above

Now, I'm confused as well. I think this is a subject in turkish grammar that cause much confusion among turks as well.
But what I remember, imperative could be just "otur" and "oturun" because when you give command, you have to directly address that person, you directly talk to that person or people, so it's just for "sen" and "siz".
I remember this from my old school days like this, but I may be wrong. This afternoon I'll check my grammar book at home.

Greetings,
I will give you a full list. I revised the last version of winmekmak and I made some changes on imperative and optative tenses. Be sure you are using version 0.20 because it is different than the previous one.

Oturmak: to sit

Dilek Şart Kipi - Conditional Tense

ben otur-sa-m If I sit
sen otur-sa-n
o otur-sa
biz otur-sa-k
siz otur-sa-nız
onlar otur-sa-lar

İstek Kipi _ Optative Tense

ben otur-a-y-ım Let me sit
sen otur-a-sın
o otur-a ²
biz otur-a-lım
siz otur-a-sınız
onlar otur-a-lar ²
²) Not recommended form.

Emir (Buyuru) Kipi - Imperative
ben ---
sen otur! Sit!
o otur-sun!
biz ---
siz otur-unuz! < here oturun should be included
onlar otur-sun-lar!

"oturun" or "oturunuz" are both second person plural imperatives. The former is the regular one and the latter is the polite one. Currently only "oturunuz" is included and "oturun" is not. I updated other details but I have missed this one. They both should be included:

siz otur-unuz!
otur-un!