Gibbons also has a Gulf States catalogue though it's been out of print for some time. Chances are they don't have all possible illustrations as like Scott, Gibbons sometimes exclude issues which they deem "above/beyond" the postal needs for a given country.
Now I do not know if this will reveal the designs you are seeking, but there is much on the SCF about Ajman. I suggest using the search function button [see row of buttons above, third from right]. One long thread return is: http://goscf.com/t/8164&whichpage=2#229286 with many photos on 6 pages.
The Michel catalogs are the best in my opinion. Mine is Gulf States in English from 2006 with a yellow cover. There is also a newer edition in a red(?) cover. Since the last Trucial States date from 1973, if you don't care about Saudi Arabia, or the UAE etc current stamps or values, these will fit the bill.
I also have a little Minkus Trucial State catalog from 1975-76. It is ok, and sometimes is better than the Michel catalog. But for comprehensiveness, go with Michel.
An added benefit for the Michel catalog are the catalog numbers which can help you buy the stamps from a foreign dealer or Delcampe (assuming you can still get on)! Plus, Steiner used the Michel to format his pages.