Autism / Autism Spectrum Support Group
Autism is classified as a neurodevelopmental disorder which manifests itself in markedly abnormal social interaction, communication ability, patterns of interests, and patterns of behavior. Although the specific etiology of autism is unknown, many researchers suspect that autism results from genetically mediated vulnerabilities to environmental triggers.
I don't have any particular book I can recommend, but in addition to any book you find on improving communication (or social skills, since they're related) you might try to find a social skills group in your area. Often the local Regional Center or Autism Society will be able to point you in the direction of one or more. That might give you a safe space to practice whichever skills you want to improve without offending your girlfriend.
Best wishes.
Autistics' Guide to Dating: A Book by Autistics, for Autistics and Those Who Love Them or Who Are in Love with Them
by Jody John Ramey (Author), Emilia Murry Ramey