To cite the greenbook package in publications:

  Coverdale C (2026). _greenbook: HM Treasury Green Book Cost-Benefit
  Analysis Primitives_. R package version 0.1.0,
  <https://github.com/charlescoverdale/greenbook>.

A BibTeX entry for LaTeX users is

  @Manual{,
    title = {{greenbook}: HM Treasury Green Book Cost-Benefit Analysis Primitives},
    author = {Charles Coverdale},
    year = {2026},
    note = {R package version 0.1.0},
    url = {https://github.com/charlescoverdale/greenbook},
  }

  HM Treasury (2026). “The Green Book: Central Government Guidance on
  Appraisal and Evaluation.”
  <https://www.gov.uk/government/publications/the-green-book-appraisal-and-evaluation-in-central-government>.

A BibTeX entry for LaTeX users is

  @Misc{,
    title = {The Green Book: Central Government Guidance on Appraisal and Evaluation},
    author = {{HM Treasury}},
    year = {2026},
    organization = {HM Treasury},
    url = {https://www.gov.uk/government/publications/the-green-book-appraisal-and-evaluation-in-central-government},
  }