。
I read a book while walking.
Literal
Walk-while book [obj-を] read.
~ながら chains two simultaneous actions performed by the same subject: 'while walking, [I] read a book.' Notice 読んだ in plain past — no politeness marker, just a flat factual report. In real life this is a slightly unsafe thing to do; pedestrian distraction by reading or phones is a common urban hazard, hence the prevalence of warnings against it on signs and in safety campaigns in Japan.