Iceland would probably count. It left its union with Denmark in 1918 at the end of a peaceful independence movement. However, it continued to share Denmark's king as head of state until 1944, when Nazi occupation of Denmark made that problematic.
The Icelandic independence movement was peaceful from its start in the
post-Napoleonic period to the ...
I'd include the partition of Czechoslovakia into the Czech Republic and Slovakia. According to Wikipedia,
The separation occurred without violence, and was thus said to be "velvet", much like the "Velvet Revolution" that preceded it, which was accomplished through massive peaceful demonstrations and actions. In contrast, other post-...
I think there is one such book that is somewhat closer to what you are expecting from a history book. The Histories of Nations: How Their Identities Were Forged.
From the description given under the book.
Histories of Nations tells the stories of 28 countries ranging from
ancient nations like Egypt and China to the US, Sweden and Ghana. The
book is a ...