I really feel sorry now I didn’t buy the copy for myself, too. Fundamental and full-scale course of the Belarusian language. Although it is formally for high-school students, it is perfect for systematizing the language.
If your native is Russian or Ukrainian, the course is easy to start without any preliminary adaptation.
English title: S. Cybuĺskaja “Belarusian Language for High-School Students”
Original title (Belarusian): С. Цыбульская “Беларуская мова. Падрыхтоўка да ЦТ
First published in the Levinarium Telegram Channel (now closed and deleted)