A 30-second FB Ad Tweak That Could Cut Your Cost Per Unit by 29%
If you run Facebook ads for your books, odds are you leave the description field blank. It’s been considered best-practice for years. It's that little text block underneath your primary copy that most advertisers — myself included — just skip right over.
But, times a-changing as they are, I recently decided to test what happens when you fill that field with an AI-generated, keyword-loaded description of the book. Here’s an example:
A second chance / small town romance with a curvy bookshop owner, grumpy x sunshine, forced proximity, one bed, a possessive hero who secretly writes her love letters, open-door steam—happily ever after guaranteed.
The idea is that maybe a strategic, trope-stuffed description can work like an Instagram hashtag and help the algorithm target users for the ad. I ran the same test across four different author campaigns using the same setup: identical creatives, identical audiences, identical budgets. The only difference was whether the ad had a description or not. Two books improved - most notably the description for the Book 2 UK campaign improved the Cost Per Unit by an impressive 29%. For the other two books, though, the ads without the descriptions in them won soundly. Same strategy, same setup, completely different outcomes.
The honest answer is the data is promising but inconclusive. I only tested one description per book, which isn't enough to draw hard conclusions. My next step is testing 5+ different descriptions per book to see whether the right description can consistently beat the control of no description at all.
But here's the thing — this is a free, 30-second change. Ask ChatGPT or Claude to write a keyword-rich description based on your blurb, drop it into the description field, and test it. Worst case you learn something. Best case you meaningfully cut your ad costs. I'll be sharing more test results like this on my site as I continue running experiments. If you want to follow along — or if you're curious about what a data-driven approach to book advertising looks like — stick around. There's more coming.
| Book | Description? | Clicks | CPC | Units | CPU |
|---|---|---|---|---|---|
| Book 1 | No | 1,547 | $0.29 | 29 | $15.50 |
| Book 1 | Yes | 1,293 | $0.27 | 18 | $19.16 |
| Book 2 (UK) | No | 3,531 | $0.09 | 113 | $2.95 |
| Book 2 (UK) | Yes | 2,092 | $0.09 | 93 | $2.09 |
| Book 2 (US) | No | 2,172 | $0.15 | 76 | $4.43 |
| Book 2 (US) | Yes | 4,875 | $0.15 | 182 | $4.01 |
| Book 3 | No | 1,788 | $0.06 | 20 | $5.63 |
| Book 3 | Yes | 958 | $0.12 | 7 | $15.70 |