The definitive reference is said to be the Spielberger book:
I've found it very disappointing as it only has a short section on each Ausfuerung and goes into a lot of detail about the later specialised applications. Expensive for little info.
I can't praise this book highly enough, although it's very expensive
It contains a wealth of detail on 10 Pz Div's campaigns in France, Russia and Tunisia. I'm particularly drawn to the latter and have a particular vehicle in mind, although I've yet to find photos of it.
A stronger claim to be the definitive reference is the Trojca book:
It is excellent, full of detail and pictures with some stunning colour schemes, particularly for Russia.
This book on Panzer Tactics has a lot of photos of Pz IIIs and is also full of good material about how the vehicle was operated.
There are a number of books with photo galleries of preserved vehicles:
This one by a Greek publisher concentrates on the Ausf F at Saumer, the Ausf L at Bovington and the Ausf M at Munsterlager.
The Osprey book is pretty superficial. It gives a good potted operational history but then so do a lot of other books.
The best of the photo books is probably the Achtung Panzer volume:
It needs to be cross-referenced with photo sources to check accuracy in some places but it does give good drawing sequences of the development of various aspects of the tank, in a way no other publication does. I'd say a must have.
This book under the Fotosnajper banner is excellent and has the best three view drawings. They are very accurate and are combined with photos of preserved vehicles and historical material. Very good value.
This book on the J/L Ausf is the best coverage of detailed photography of preserved vehicles. Of course, care needs to be taken as some features of the vehicles in museums are later additions. Nevertheless, if you don't have access to a museum vehicle yourself, it is a very good source.
The next four are self explanatory and contain historical material.
Finally, two references aimed at the plastic scale modeller, which contain some useful material.
I hope you find this useful and that more contributions are posted. If anyone needs an ISBN, please ask.
Regards
Stephen
If I had something to add I would but the few PIII books I have are already there. Looks like you have everything you need to build an accurate model.