"Buku ini menggambarkan tentang pemerintahan Jepang di abad lalu."

Translation: This book describes the Japanese government in the last century.

[RonLawler1]

Japanese government without "the" in front of it should be accepted as there are many governments in the course of a century or if pemerintahan is translated as "governance" then "the" is certainly not needed

[glhaynes90]

What is the difference between pemerintah and pemerintahan in this context?

[joeldipops]

https://kbbi.web.id/perintah

I'm not going to try to translate any of those huge blocks of text but the important distinction seems to be

pemerintah: sistem ... mengatur kehidupan sosial, ekonomi, dan politik suatu negara atau bagian-bagiannya;

System of organising society, economics and politics of a country or part of a country.

pemerintahan: proses, cara, perbuatan memerintah
The process, method or action of governing.

So I guess the second could be translated as "Governance" rather than "Government"

[Bev425285]

Why is tentang necessary?

[Bev425285]

"The past century" should be an accepted translation.