Well, the first place to start would be to figure out what kind of things you'd like to invest in.
If you're just talking about the securities exchanges, there are some excellent reads out there. I've never really done much with it myself, because I prefer tangible assets (Real Estate). I'd highly recommend Nassim Taleb's Antifragile
, for investment, and for life in general.
It's less of a direct guide to investing, and more of a guide to changing your mindset so that you avoid engaging in behaviors that put you in the position of the "Sword of Damacles" myth (if the thread breaks, you're dead) and not even that of the Pheonix (no matter how bad things get you bounce back), but rather that place you in the position of the Hydra (cut off one head, two take it's place). The more things go wrong, the better you do, and if things don't go wrong, you're not any worse off than you were.
Some other folks may have some great ideas for investment books that are more of the how-to guide variety.
But JLD, you also don't have to just invest in securities, there are other things a SAHM can do to make money. Starting a part-time home business is always an option. I know folks who go to auctions, and buy stuff for resale on the interwebs for example.