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InfluxDB
Power Real-Time Data Analytics at Scale. Get real-time insights from all types of time series data with InfluxDB. Ingest, query, and analyze billions of data points in real-time with unbounded cardinality.
It's not a commercial off the shelf product, but the Open Book uses a 4.2" screen. There are other devices you can find on places like AliBaba that are kinda small. However, in general you won't find a name brand commercial ereader under 6" that isn't ina phone body. There were a couple at 5" back-when but the industry really settled on 6" as the common base size.
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