The best source I could guide you to is a fantastic work, titled Putin’s World. Apart from that, Putin has had a lot of televised and radio appearances that work on his immediate thinking. But this book provides a fairly complete picture, citing Putin extensively throughout.

His mentioned 2021 essay gives a good view of his perspective toward Ukraine, but I recommend speeches surrounding his annexation of Crimea in 2014 as well.

Hopefully, that helps!