It is cool to have an unequivocally authentic Brian May autograph and £70 is worth it i.m.o. One of the few guitar players in the world that impressed Frank Zappa - not that there is any link to that other world of his.
Be aware, though, that the books are sealed in a slipcase and the bookplate is delivered loose inside the package (it is not actually laid-in, i.e.).
But: Couldn't Brian May just sign a Queen photo book? That would be an instant purchase – while the books about optical phenomena are interesting in their own way, they have little to do with the reason for his popularity...