Since you already have several lightweight WordPress sites running successfully on shared hosting, I’d lean toward keeping the new book site as a normal addon-domain WordPress install.
Multisite can centralize management, but it adds server/configuration considerations and can make migrations more complicated. WordPress also notes that shared hosting may require the host to support specific Multisite requirements.
For a low-traffic, mostly text-based site, a separate installation keeps things simple and isolated while avoiding the extra complexity of migrating your existing sites.
