Below is the book by Charles Kiddle, OWLS: The Story of Poster Stamps. This catalog was produced for the U.K.’s BIRDPEX exhibition in 1998. The text was intended for non-poster stamp specialists, however even experienced cinderella collectors can be surprised at the large number of stamps that takes almost 60 pages to cover. This spiral bound book has some notes giving a catalog numbering list and estimated prices on the back pages. This flip book is in color, however the first printings were in black and white and may be found.
At the time of posting this catalog, a related article on one particular design, “Stender’s Owl” explains more fully the source and variations of that design (as shown on page 18 above in Owls.)