You are right it is tied to Thieves' Guild, but not Skyrim's one.
You see, this book is originally from Oblivon and this tale (fictional is my guess, but still...) portrays how centuries ago Emer Dareloth stole Nocturnal's Cowl and became The Gray Fox. So it ties to Oblivion's Thieves Guild, where you basicaly work for Gray Fox and eventualy become The Gray Fox, wearing Nocturnal´s Cowl.
You are right it is tied to Thieves' Guild, but not Skyrim's one.
You see, this book is originally from Oblivon and this tale (fictional is my guess, but still...) portrays how centuries ago Emer Dareloth stole Nocturnal's Cowl and became The Gray Fox. So it ties to Oblivion's Thieves Guild, where you basicaly work for Gray Fox and eventualy become The Gray Fox, wearing Nocturnal´s Cowl.