Remember, too, that income from a book doesn't arrive all at once (or in a steady trickle).
Let's say you get an offer for your first novel from a publisher in January. Using average amounts, the deal is a $5000 advance, with 10% royalties on the hardcover edition. The advance is split into three parts--$1500 on signing, $1500 on delivery-and-acceptance, and $2000 on publication.
A month or longer can pass before the contract arrives. You have a speedy publisher, so you get your contract in mid-February. You sign and return it. A few weeks later, in March, the first check arrives from your agent, minus her 15% commission. You get $1275.
Meanwhile, your editor also sends you a detailed letter, describing the revisions she'd like you to make to the novel. You take three months to finish those. She reviews the revised manuscript. That might take a few weeks or a couple months, depending on her schedule. If all goes well, she formally accepts the novel. It's July when you get your second check, also for $1275.
Now comes the longer wait for the book's release. Lots of things are happening--book design, cover art, copyediting, acquiring blurbs, writing the catalog copy for the publisher's sales force, etc. Depending on the publisher's schedule, it might be nine months or longer (sometimes much longer) before publication date. Let's say your book appears sixteen months after the offer. So the next May, you get $1700.
You've earned $2550 the first year, $1700 the second year, for that book.
At this point, you get nothing more from the publisher until the book sells enough copies for the royalties to reach the amount of your advance. If your book sells for $20, and you get a 10% royalty, the publisher needs to sell 2500 copies to equal your advance.
Once your advance earns out, you get royalties at regular intervals through the year. Usually it's twice or four times a year, but your contract will spell that out. Remember that your agent gets a cut here, too.