Toso Kodogu Meiji Taikei by Wakayama. It is a three volumes book full of high quality reference photos of verified signatures of tosogu craftsman. According to some this is a book used by NBTHK judges to verify signatures on items that they evaluate. There is no better book for signature comparison. Please note that there are only pictures of signatures, so this book will not help with compering workmanship, style and techniques used.
It is in Japanese but there is an index in hiragana, that is rather easy to read even If you cannot read in Japanese.
In this set two books are in hard cover and on is in soft cover. There are some damages on the soft cover one. Two in a hard cover are in almost perfect condition.