All these letters were written by Nehru during the periods he spent at various gaols(jails). So all the reference he had were his own notes he made while reading the great books.
The book contains about 196 letters written between 1930-1933. All these letters, though not a first hand reference on history, contain Nehru's personal remarks on all the great moments of history. (elaborately on all the things he liked, in particular).
Above all, this book is about history by a history-maker and not by a just another historian.