Illegal books? Maybe the illegal meaning is just from a country that prohibits gambling. Sports betting site is not illegal, especially in a country that allows gambling. We can suggest you to use the casino that have reputation so you don't have to worry if they will cheat or scam you. Hack the site? If the casino site have a high security level, that site will not gets hack easily because their security teams will always monitor and protect the site from the hackers. So there's no way to hack the users balance unless there's a bug or hole in the site that the hackers can enter to the sites and takes the users balance. But we should aware that nothing is 100% secure so there still a chance that hacker can enter to the site and do something for their own purposes. But as long as the security teams can works to protect their site, that things will not happens. The casino will make sure their members will be safe and comfort while playing gambling in their site.