FYI: Calibre contains a program appropriately named "ebook-viewer" that allows you to view a file (like an epub) without all the library management stuff.
This already exists: applications can receive memory pressure events (such as the system reaching "medium" level, where you may want to start freeing some caches) via /sys/fs/cgroup/memory/.../memory.pressure_level. See https://www.kernel.org/doc/Documentation/cgroup-v1/memory.tx....