Rarities
The Observer's Pocket Series

As with most forms of collecting there are rarities in the Observer Series.
Within this category come certain titles, editions and jackets as well as a few special issues and oddities. 
What constitutes a rarity as always in collecting is supply against demand, but in almost all cases it is the condition of the book and jacket that is the prevailing factor. 

A note to non-collectors and dealers - no matter how scarce a particular title or edition may be, the following comments apply ONLY to books in good collectable condition with their original dust jackets if applicable, any books that do not meet this criteria have little value from a collecting aspect no matter how rare.

Titles.
Some of the rarest titles are those first published in 1980-82 era, "Paris" and "World Atlas" are considered the among the rarest of these followed closely by "Kitchen Antiques" and "Opera", very small numbers were sold and these do not turn up for sale very often. 
All other titles between nr.79 and 98 are scarce too, however these are likely to turn up from time to time. (see title list), sales of these titles were less than 20,000 in most cases.

First editions of the early titles also have rarity status, at least those in a collectable condition, in particular the first ever titles "British Birds" and "British Wild Flowers", in their rather drab monotone jackets, to a lesser degree the other titles published in the '30s and 40's are also scarce, of these "The Observer's Book of Airplanes", 1942 and subsequent editions (later to be re-titled 'Aircraft) will be really hard to find in good condition and are extremely desirable as they have special interest for their content too.  
Although most of the early Observer's first editions sold in considerable numbers, many of the surviving books have lost their jackets, been defaced or have been subject to rough handling or storage at some time of their life, rarities in this case are those few remaining in collectable condition.

From the 'fifties titles, 'Mosses and Liverworts', early editions of 'Aircraft' and 'Automobiles', are really sought after, it is hard to find good copies of the nature titles such as 'Ferns','Fungi' and 'Birds Eggs'.

Of the 'Sixties titles, 'Lichens' is much less common than other titles from this era because of the subject matter and to a similar extent so are the Automobiles and 1966 first edition of Commercial Vehicles, again due to the amount of handling these titles had.
Other first editions from the sixties barely qualify, although "Sculpture" and "Modern Art" are not too plentiful such titles as a first "Postage Stamps" or "Sea and Seashore" are really quite common.

The later seventies titles 'Jazz', 'Big Bands', 'Fossils', 'Seashells', 'Vegetables' and 'Cotswolds' are all getting harder to find, these were printed only once and most had total sales in the region of only 20-30,000 copies. 

Jackets.
Books with certain jackets are really sought after by collectors, of these the 'adverts' or Cyanamid jackets are top of the list, these were fitted to certain titles circa 1980 and given out as a freebee to customers of the goods they advertise.  Probably similar quantities were produced although some Cyanamid titles do turn up more than others.
Another two 'Advert' in this category would be the 1980 Aircraft which has a special jacket that was commemorating the Farnborough Air Show, this one is quite rare and I did have a chance of purchasing one relatively cheaply one one occasion but missed the opportunity.
Another special advert jacket that was fitted to a few of the 1974 Observer's book of Commercial Vehicles again as an advertising gimmick, I have never seen one and I guess these are extremely rare.

At the two style change points of 1954 (removal of British from title) and circa 1971 (re-styled jackets) a number of printings/editions were produced with more than one jacket variety and it is likely that one is more common than the other. 
An example of this is the 1966 single printing of Sculpture, in itself not too common but after 1971 the remainder of the same printing was fitted with the new style jacket and this is even more uncommon. Around this time three titles were re-jacketed with rather drab black and white illustrated covers, Common Fungi, Lichens and Ferns, this was a cheap option to re-jacket a few remaining unsold books no doubt,  I remember seeing one of these at a car boot sale a few years ago and thinking that the Observer's series had sunk a bit since I remembered them! (I didn't buy it either - bah).  All three are scarce, Fungi and Lichens especially so, I have never seen the Fungi b/w !

In some cases a single printing or edition was fitted with one jacket type, sometimes a small variation of the style, on the older titles like Wild Animals, Birds, Butterflies etc., these jacket types are also quite scarce.

'Glossies', is a term used by collectors describing certain jackets fitted around 1970 to some titles, these were the old style 'wavy edge' jacket but given a laminated plastic coating like the later 'white' jackets were produced with, these appear to be an interim measure to brighten up a few unsold older editions before the new type of jacket was launched.  Books with these glossy jackets are really scarce on some but not all of the titles, most common one is the Geology 'glossy' which turns up quite frequently as does the Birds 'glossy'.

There are many other scarce jacket types, some with just slight variations, there is not space to detail them all, and without pictures and the necessary detail, it could be misleading just to list them.
There are references available, including the magnificent and extensive publication "Observer's Look at Observer's Books" by Mick Burgess, which illustrates ALL Observer's jackets and covers, there is also  the various resources on the internet.