Bits and pieces relating to the Nationalist Ukrainians of the period.
Osprey have Ukrainian Armies 1914-1955. Most of it is for the 1919-1920 period.
The books to get though, provided you can read the Russian, are the books by Yaroslav Tinchenko. The old Ukrainian Armies 1917-1920 (Армии Украины 1917-1920) for example is good value.
The fighting against the Poles in 1919 is here