I have been very moved by the book Siddhartha and by other works by Hesse. However, I am curious whether there are any recommendations of books, by different authors, which deal with similiar themes - (spiritual journeys with Buddhist influences, or other) Thankyou

Off the top of my head, I can think of "The Alchemist" by Paulo Coelho.
There have also been some Beat writers that centered their poetry around Buddhism, most notably Gary Snyder (who in fact became a Buddhist monk). Also, Jack Kerouac wrote a book "The Dharma Bums" which explores Buddhism. You can also find Buddhist influences running through Allen Ginsberg's poetry.
If you can stomach existentialism I would suggest any Albert Camus novels, especially "The Stranger", though it doesn't deal with Buddhism beliefs it still explores the spirit of man, human nature, and spiritual journeys.