If we're talking about a visual and layout-focused template, Adobe InDesign is basically the designer's choice for a reason, but man, the learning curve is a wall. I messed around with it for a project once and spent more time watching tutorials than actually making my template. For most people who just want a nice, consistent look for their personal reading journal or a book they're formatting themselves, I'd actually lean toward Canva. It's way more forgiving, has tons of pre-made elements you can drag and drop, and you can create something pretty polished without any design background. The free version is surprisingly robust.
That said, 'customizable' can mean different things. If you're thinking more about tracking your reading data—like progress, quotes, notes, ratings—then a tool like Notion is unbeatable. You build your own database, set up properties for genre, page count, start/finish dates, and you can view it as a table, a gallery of covers, or a calendar. I built one last year and tweak it constantly; I've got linked pages for each book where I paste in my chapter notes. It's less about a pretty printout and more about a dynamic, personal library system that works exactly how I want it to.
The real niche option, though, is Scrivener for readers. Sounds weird, right? But its binder and corkboard view are fantastic for deep analysis. You can import an ebook (if you own it), split it into chapters or scenes in the binder, and then use document notes or the comment feature to annotate right there. You can compile all your notes into a separate document afterward. It's overkill for casual tracking, but if you're studying structure or writing a review, having the text and your observations in one project file is a game-changer. The customization is in the workflow, not the visual template.