UK “Half-Blood Prince”: 608 Pages
Potterish.com (Portuguese) scanned a Bloomsbury form letting UK booksellers know they are now able to order cartons of Book 6 to be sold on July 16. The scan lists the book’s length as 608 pages. The US version of Half-Blood Prince is 672 pages, but both versions use a different font.
Thanks to Harry Potter Fan Zone via Night Elf for the tip!