DMG page 8:
As this book is the exclusive precinct of the DM, you must view any non-DM player possessing it as something less than worthy of honorable death. Peeping players there will undoubtedly be, but they are simply lessening their own enjoyment of the game by taking away some of the sense of wonder that otherwise arises from a game which has rules hidden from participants.
It is in your interests, and in theirs, to discourage possession of this book by players. If any of your participants do read herein, it is suggested that you assess them a heavy fee for consulting "sages" and other sources of information not normally attainable by the inhabitants of your milieu. If they express knowledge which could only be garnered by consulting these pages, a magic item or two can be taken as payment - insufficient, but perhaps it will tend to discourage such actions.
PHB page 8:
THE REFEREE IS THE FINAL ARBITER OF ALL AFFAIRS OF HIS OR HER
CAMPAIGN
Captialisation as per the book.
Therefore, ALL GM statements stand.
Blank spell books are NOT commonly available. The materials usually are however.
Therefore somewhere is needed to work in to create the book from the materials and then transcribe spells (1 day per spell level I believe). This is not something that can be done while on the road.
Also, as said last night, there is NO WAY full value will be received for the books. Nobody besides small countries and larger have the available funds. Remember, the vast majority of people are lucky to be paid a few gold in a whole year!
Bracers and rings do combine. Bracers, rings and shields do not.