First thing to address is masons will not kill anybody, mason or not, secrets known or not. Freemasonry is a benevolant fraternal organistion not a mob.
Secondly the book you mention is not a secret book, it is available via Amazon, and most Grand Lodge book shops. I purchased a re-print of this very book for £20 (about $40) from the United Grand Lodge of England book shop, before I was a Freemason and there were no questions asked.
The book doesn't contain rituals, but does have some explination as to the symbolisms and certain elements of lodge life (opening of a lodge, constituting a lodge, the masonic charges, and some of the emblems). It is also specific to a specific as the full title of the book denotes,
"New Masonic Trestle-board Adapted to the Work and Lectures as Practiced in the Lodges, Chapters, Councils, and Encampments of Knight Templars in the United States"
It is specific to America, where today most lodges do not have trestleboards anyway, and much of what is contained in the book is no longer done or taught (shame though)
The only symbol styled text that would be used in Freemasonry is either Hebrew, which is used on some designs, specifically those for the 3rd degree although older ones contain more of it, or cyphered text which is letter replacement.
The book is legit in the sense it's information was once current, however as mentioned, the US don't utilise the trestle board anymore therefore the information is more historic fact than anything else.
It also reveals no secrets, or anything secret, from within the masonic rituals. The only secrets are the modes of recognition which are not detailed during any element of Freemasonry when trestleboards are used. We do not discuss much of what the book contains however, not to keep it secret, but so not to spoil the experience for any new candidate. Would be a bit like telling someone how a film ends when they've just bought a ticket to see it for the first time.