Most of the mythology is lost in the sense that there are no written primary sources from the Magyars of the time. However, researchers have been able to reconstruct some of it through secondary sources, archaeological evidence and folklore. Therefore, there is no unified mythology and multiple theories exist. The best resource I have found so far is the book “Magyar őstörténet és tudományőrzés”, specifically the chapter from Hoppáll Mihály on ancient hungarian religion, it summarizes the research that has been done so far and provides a lot of sources for further reading both through the citations and by naming prominent scholars whos work you can look up. The book is also made available as a free pdf by the Hungarian Academy of Sciences (Magyar Tudományos Akadémia).