My copy of Invitation to a Beheading starts with a foreword where Nabakov specifically denies any connection to a number of authors, singling out Orwell and Kafka in particular for no denial. Nabakov is a writer who is particularly unreliable in what he says about his own work, and Orwell and Kafka are the correct comparitors here. Th book is very much like Kafka and has obvious affinities with The Trial. Nabakov denies having read any Kafka at the point he wrote Beheading and there is no reason to disbelieve him, but this book is trying to do something very similar. Both depict a world where someone has been charged with a crime whose very nature is obscure. In both the worlds are bizarre and irrational. Both are concerned at least in part with the irrational forces then gripping the European continent. Of the two, however, Kafka is by far the best, and while Beheading is worthy, it is not as good as The Trial or as Nabakov's later works.
Along with Orwell, Nabakov is one of the two great writers about oppression of the twentieth century. While their styles are very different, there views on oppression are virtually the same. Nabakov is not at his best here. The bizarre word interferes with his depictions of the delusional nature of oppressors and he is always at his best when writing from the oppressors vantage point rather than the oppressed. However, Nabakov is still a great writer and while this only prefigures what is to come, its well worth reading, presuming that you have already read The Trial and Lolita.
Good but not great.