I am trying to find articles or maybe a book that covers the "Minor" Axis Powers: Hungary, Romania, Slovakia & Bulgaria (or just one of them). The Cambridge History of the Second World War Vol. II has chapters on the "non-belligerents" Spain & Sweden, yet, sadly not the "minors".
I know Third Axis Fourth Ally: Romanian Armed Forces in the European War by Mark Axworthy, but I don't have access to it anymore.
I have read this book - The Unknown Eastern front “ by Rolf Dieter Muller..it covers Finland,Hungary,Romania,Italy,Solvakia,Croatia, plus many others. This is the link: https://www.amazon.com/Eastern-Front-Wehrmacht-Hitlers-Soldiers/dp/1780760728/ref=sr_1_1?ie=UTF8&qid=1521546575&sr=8-1&keywords=the+unknown+eastern+front
For Slovakia, The Parish Republic by Yeshayahu Jelinek, also Priest, Politician, Collaborator (a biography of Jozef Tiso, leader of the Slovak People’s Party and the wartime Slovak state) by James Ward, and At the Price of the Republic by James Felak (about the People’s Party, although it ends in 1938).