The back cover is not leftover space. It is a small reading surface with a sales job, a production constraint, and far less room than the same copy appears to occupy in a document editor.
Give the hook a separate job
A short hook can create curiosity before the reader commits to the blurb. Keep it brief, visually distinct, and subordinate to the front-cover promise. Repeating the title or adding a generic sentence only uses space that the description needs.
Set the blurb for reading
Use a comfortable line length, sufficient leading, real paragraph breaks, and a print-safe size. Avoid placing light text over detailed or high-contrast artwork. A calm overlay or extended texture can maintain continuity with the front while giving the copy a stable field.
Reserve production space first
Mark the provider's barcode zone before positioning any ornament, logo, quote, or text. Keep important elements away from trim and fold tolerances. If the printer supplies a barcode, do not treat its reserved rectangle as optional decorative space.
Proof at actual size
Zooming into a large screen hides how small the copy will become. Print a paper proof or view the PDF at measured physical size, then check line length, contrast, paragraph rhythm, and the visual path from hook to blurb to supporting details.