Don't buy books; there aren't many geared toward new investors and most of them are shit. Investopedia is a good place to start on the internet.
Easiest way to get started without much involvement or knowledge on your part: A service like Betterment or Wealthfront. Sign up for account. Put money in. They automatically choose a collection of high quality index funds for you. The only involvement on your part is to choose your ratio of stocks to bonds and to make sure you tell the government about any money you make. They want their share.
You'll pay a bit extra to Betterment or Wealthfront, but their fees are quite low and its a good choice for beginners. You can always transfer your accounts to another service later.
If you want something more advanced than a robo-advisor (Betterment/Wealthfront), you can try buying index funds yourself. Portfolio allocation is a whole other topic, but here are some of the more recommended brokers and why you might or might not choose them.
Vanguard: The brains behind some of the most highly recommended ETFs out there (though you don't have to have an account with them to own their ETFs). You can trade any Vanguard ETF or Mutual Fund commission free, but to trade anything else is horribly expensive ($20/trade). Fortunately you can create an awesome portfolio just with Vanguard ETFs.
TDAmeritrade: Access to a great collection of ETFs commission free including most of the Vanguard ones you'd want, a lower (but still kinda high) $10/trade commission for anything else, and hands down the best tools (and user experience) of any broker. If you thought financial institutions couldn't make a decent mobile app, go download thinkorswim and have your mind blown.
Fidelity: Commonly recommended around here. Commission-free ETF list only includes Fidelity and iShares ETFs, but you can make a great portfolio out of those. All other trades are an even lower $8/trade. I'd personally go with TDA just because if you're trading ETFs anyway then the commission doesn't matter and the tools are worth it.