Ebook reader with a lot of local storage and downloaded content.

If the grid is down, nothing but local storage will work.

A smartphone can potenttialy be quite useful even without cellural/internet connection. Entire wikipedia can be downloaded for offline use and many maps too. I'm sure there are plenty of survival apps that work offline too that contain a ton of useful info about edible plants, shelters, traps, plant medicine, knots etc.

None of this is useful though if you haven't downloaded it before the apocalypse and have some kind of way of charging your device.

Ideally you'd want an android tablet with an e-ink screen and expandable microSD storage loaded up with textbooks.

Something like this

As far as collecting immediately useful intelligence of zombie movements, I think a foxhole radio would be more maintainable than a cellular phone.

spoiler

If you're lucky, you can find a ham radio operator with such a device. You could use the radio to coordinate with other distant groups who may have resources like the smartphones you are referencing. Yes, smartphones are technically radios, but I donʼt think the required cellular towers can be relied upon during an infrastructure collapse.