An agent recently posted a blog entry about how much value can be earned by being represented during a book sale. There’s no doubt that a professional negotiator can keep dollars in your checkbook while you arrange a contract with a book company. As good examples, Rachelle Gardner lists e-book payments, frequency of royalty checks, sales threshold for royalties to begin, and size of advance. All, she says in her Why Authors Need Agents posting, provide value in exchange for an agent’s cut.

But there is that cut, since everybody gets a piece of the author’s efforts when a book is sold to a publisher. 15 percent in most cases, making the average first-time author’s $5,000 advance a $4,250 check to deposit. Nobody should quibble over $750 in service to publishing a book. You’d do well to spend that little on an outside professional edit before submitting your work to anybody. Even if you’re publishing it yourself.

There’s another factor in the formula to consider, and that is the sale itself. Agents are paid sales professionals whose primary function is to interest an acquisition editor in your book. What caliber of publishing company, and what treatment your book gets after signing the contract — well, these are benefits whose value varies wildly. The first benefit changes all the time in these days as publishing firms scramble to stay profitable and in business.

The second benefit shifts according to who remains in the publisher’s employ. There’s a lot of job shifting going on out there. You can have a good editor and lose her, or get a better replacement who will need to learn your book to be of any help. Gardner’s most salient point is that a writer who has little interest in participating in the publishing business gets to focus on their creative craft when they employ an agent. Again, it’s true with a caveat: don’t be thinking you’re just going back to the keyboard after the sale for revisions from an editor’s notes, then onward to the next book.

Few publishing deals leave the sales effort up to the force entirely. The force may be with you, young writer, but you must practice its ways in any arrangement. You can get a turnkey deal. A friend of mine is writing a remarkable book under a very nice contract with a publisher well-known for its adept sales force. She’ll do readings, of course, be interviewed and the like. A lot of what she’s earning from the book is either already accomplished — the strength of her work that won the deal — or expected from the publisher. Many deals are not as fortunate.

We write for many reasons, but an important one is to be paid enough to keep writing, to be an author of several books rather than someone who wrote a book. Most of the time that requires continued effort to promote and interest the world in your stories. A great author Web site, Twitter, and to a lesser degree Facebook, are ways I see writers taking the reins in creating a platform for their voice and their work. An agent might be able to give advice about this platform work, but you would hope they’re working harder on advice about making your manuscript salable, or finding a buyer for your book.

Gardner wants us to believe that every standard publishing contract contains benefits from prior work of other agents — sort of like we’re supposed to believe we’re indebted to Louis Pasteur when we get an H1N1 vaccination. It’s a stretch in science and kind of disingenuous in examining agents’ value. Agents are performing services today that publishers once did. Editing, for example, in the scope of showing a writer how their story could be better, and so sold sooner and at a better price.

Publishers with good editors are getting rare. Even my friend’s book is mostly bereft of a close relationship with an editor. Considering that the fee for a good edit and the agent’s cut of your average advance are similar numbers, the value of being agented seems on par with being edited. A well-written book makes everything richer. Sales specialists spin the threads of your work into the gold of folding cash. But you need to ensure your threads are in their best order to even get a reading from an agent.