This doesn’t list the actual comorbidity numbers for it, but does indicate NPD and schizoid comorbidity in some: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2669224/
I can’t read this one, but it discusses schizoid and narcissistic features as complementary: https://pubmed.ncbi.nlm.nih.gov/10601823/
Haven’t checked if this author is legit, it’s too hard on my phone lol: https://www.amazon.com/Lonely-Recluse-Narcissist-Narcissistic-Personality-ebook/dp/B00HU5HMG8