I don't recommend Joe Ross.
Courtney Smith's "Futures Spread Trading" is a reasonable introduction. He doesn't try to sell any BS or pretend that you can just jump in and make easy money, he just explains the basics of what drives spread prices and how you might go about trying to trade them. He has also worked in the industry for many years, so at least has some credibility (although that doesn't mean he can trade spreads well).
Beware of anyone marketing a get rich quick system, or even a "steady income with little risk" system. You learn about spreads the same way you learn about anything else in the markets - sit down and watch them day after day, week after week, month after month. Out of market hours, read everything you can about them, in books, internet articles, magazine features etc (be aware you won't find much, and a lot will be erroneous). Try to get employment with a firm or trader who trades spreads, and pick their brains. Learn the theory of fair value for spreads (mostly arbitrage value based on cost of carry, although for financials it is more complicated), and learn the fundamental factors that can make spreads blow in or out. Watch how spreads get out of line temporarily during the day due to order flow, watch how they come back. Watch very carefully those times when the spread blows out and *doesn't* come back. Then start trading with one lots, and do not increase your size until you have a few months of fairly consistent results.
If you do all this, you might eventually be able to trade spreads profitably with reasonable risk. But you won't IMO learn it just by reading some course.