As already stated, the book will be your best single source to study the set. There is a chapter dedicated to set rarities including a list of the top 100 most difficult subjects to obtain. The book is rich in photography with over 2,500 images allowing you to study the set in detail not previously possible.
If interested in a copy, there are a couple options outside of Amazon including California Sportscards, sometimes eBay, or even from Richard Masson himself in a pinch. Here is one that ended a couple days ago, I'm sure another will be listed before long.
http://cgi.ebay.com/Old-Judge-Photo-...item2c5274b239
California Sportscards:
http://www.californiasportscards.com...-p-324679.html
Back to your original post, the most difficult players to obtain are the 19 catalogued members of the California League. Some of the cards are unique with all but Veach having only a single Old Judge pose (Veach also has some tough poses with St. Paul). I should note that McDonald also has a second pose as a Gypsy Queen portrait (a unique card). Outside of the California League cards of 1889, many of the sets rarities reside with some of the players only issued 1888 Fb cards and 1886/87 Script cards. If you extend rarities to individual poses or even team change variations, the list of unique examples grows considerably including some of the 1890 NL/PL cards. My favorite unique pose is the portrait of Deacon White (496-9).
Kevin Cummings listed most of the poses I find interesting and silly. One that I don't believe has been mentioned yet is the "Long & Short" of Nicol & Reilly.
Barry, how could you miss the Horner / Warner card, you proof read an early draft for us?
That is a rare two player card you don't see everyday. I too like the composition of Milwaukees pitcher and cather.