From the longer Wikipedia page  - there is more information on the Russian and Ukrainian pages.
The Book of Veles (also: Veles Book, Vles book, Vlesbook, Isenbeck's Planks, Велесова книга, Велесова књига, Велес книга, Книга Велеса, Дощечки Изенбека, Дощьки Изенбека) is a literary forgery claimed to be a text of ancient Slavic religion and history purportedly written on wooden planks.
It contains religious passages and accounts of history interspersed with religious morals. The earliest events in the book could be dated around the 7th century BC and the latest happened in the 9th century AD.
The book was allegedly discovered in 1919 and lost in 1941. It is widely held by scholars to be a forgery made in the 1940s–1950s or, which is less likely, in the early 19th century. The main decisive evidence is the language of the text which is a mixture of different modern Slavic languages with erroneous and invented forms and no regular grammar. Moreover, different modern editions of the book have different versions of its text. Regardless, some Slavic Neopagans use it as their sacred text.