This is a syslogd implementation that is the answer to the question "what if the busybox developers made journald".
This is a tiny daemon that receives kernel log messages and syslog messages on an unix socket and stores those in an sqlite database for querying. Due to how sqlite works it's possible to run a persistent syslog or an in-memory one (which is the default)
The logbook contains a copy of the full kernel log since boot and gaps are automatically filled when restarting logbookd as long the lost messages are not more than the size of the kernel log ringbuffer.
To read log messages the
logread command will query the daemon
over another socket and it gives the possibility to see
realtime (filtered) log messages.