After seeing the film, you'll be curious to read Graysmith's non-fiction book. I read it years ago and the strange, horrifying details of the case still haunt me. Despite Chloe Sevigny finding it “too tedious”, I think you'll find the girth of detail engaging and immersing. A classic in true crime.
If previous pick Helter Skelter was the ultimate court case account, this is the ultimate unsolved case story. It's also interesting to go online and see new theories (including Scientology and Manson conspiracies, complex radian theories, and other possible suspects), especially since, despite compelling circumstantial evidence, there is no physical (fingerprint, handwriting, DNA) evidence to confirm it was Arthur Leigh Allen, as the book suggests.