This is a very valuable first edition book by Clarence Hailey from the early 20th century. The title Thoroughbred Sires is only in three libraries Worldwide and is seen on the Worldwide markets but not often. This collection taken from pictures by this known author on horses from the period, contains, as you note, 100 engravings of "thoroughbred sires; with their tabulated pedigrees (at end of book)"
Currently you can find some trying to get more for the edition but the true market value of the book is in the range of 500.00-600.00 (USD) when you consider past sales of similar items documenting horses from the last three years as well as high end auction sales of similar material in recent auctions.
Here is one record from New Zealand but I was unable to find what it sold for - I believe it is a bit low in the estimate looking at the market and how many are out there. This shows to that it is not completely rare but scarce which is good to keep in mind.
Lot 206: [ Books ]
Hailey, Clarence. thoroughbred Sires the Collection contains 100 engravings of Thoroughbred Sires, with their Tabulated Pedigrees (at end of book), 1st Ed, Clarence hailey, Newmarket, 1908, full green morocco gilt, covers worn and marked, FX, else G.
Dunbar Sloane: Wellington, New Zealand
Auction Date: September 30, 2009
NZD300 - NZD500
All my best, Dori
This is such a fine book that I do not think that the small notes would make any difference in the value..if they could be carefully erased (pencil of course) I would make an effort to do so without damaging the paper..but nothing else...especially if it is pen.
As for the place to sell this book the best places would be Christies, Sothebys or Bonhams but a place such as Mullocks that has sporting auctions and sporting rare book sales maybe a very good option as well. The market would be tighter and possibly the interest stronger in this type of venue. http://www.mullocksauctions.co.uk/
Dominic Winter has done well with rare books on horses as well as Bloomsbury Auctions, call and see which suits you best..and is of course closest to your location.
Best wishes, Dori