It's not exactly "free", as you have just exchanged your email address in return for the book. The book itself is probably nothing more than "basic strategy" for gambling, as the REAL "secrets" are the preserve of a small number of "advantage players", and they will NOT want every Tom, Dick, or Harry using them, as this will get the attention of the casinos, who will then change the rules to render the methods obsolete.
There are going to be a few of these books that really ARE worth having, and they may even be supplied for free, with no strings attached.
One such site, although not a book, is the one run by the Wizard of Odds, a skilled mathematician who specialises in casino mathematics and probabilities, and even designes and asseses new games for the industry. This has ALL the strategy information you need for a large number of games, and is even divided into the different online softwares, along with downloadable strategy cards. This is all for FREE.
His site also has the mathematics behind "bonus abuse", showing just how and why some very odd looking strategies actually work.
The only thing an ebook can cover is tips from the years of experience of a successful player, and an HONEST one. This is something you will NOT learn from the maths alone, and would cover aspects like understanding terms and conditions, how to avoid the common pitfalls that lead to problems getting paid, the difference between bullshit and fact, etc...
This is covered in sections of THIS site, and is again for FREE.
The forum here, and in many other sites, offers the combined experience of MANY players, and again mostly for free.
When I was still mainly into Fruit Machines, I joined a couple of paid-for sites that advertised "winning secrets". I had the advantage of knowing a few REAL winning secrets, and NOT ONE of these got a mention on either site. What I DID find were the basics that might help a complete newbie, plus a few OLD secrets that had long since been "chipped" out of the machines by the suppliers, so of course no longer worked.
The secrets that still worked were NOT there, because they were being carefully guarded by those that knew them.
Look what happened when "neworleans" posted the RTG "Frozen assets method" on the forum, one that had worked for YEARS. In a short few months, RTG "chipped" the product, and the game was removed, along with a number of others.
If such a method still exists and works, for any online software, you will NOT find it in a free ebook, and probably not even in a paid for one, until it is more or less "dead".