How do I change the design template of my book? The gallery of designs is no longer on the page.

On the book's page (version 1.9.19 and later), open the "Book design" card in the right column. It shows the design in use, with a small preview in its real fonts, and a "Change design" button. That button opens the whole gallery in a window: Classic, Elegant, Modern, Minimal, Vintage, Storyteller, Scholar, Romance, Noir, Ming and Custom. Click one and it is applied at once. The trim size and "Page preview" stayed on the card under the design. The gallery used to be laid out on the page itself, which made the column very long; only its place changed.