The ISBNs (International Standard Book Numbers) of Thomas and Webb, Butterflies of Dorset, are seriously muddled.
The UK ISBN Agency's database shows
0-900341-20-3 978-0-900341-20-5 Butterflies of Dorset (hardback)
0-900341-21-1 978-0-900341-21-2 Butterflies of Dorset (paperback)
The back cover, of both editions, shows
0-900341-20-3
The verso of the title-page, of both editions, shows
0-900341-21-1
0-900341-20-1 pbk (an invalid number)
Correcting this complex error would be difficult, and as it has stood for some 30 years without apparent problems I have left it alone.
John Palmer, 2014-02-05