If you would like to cite the Environmental Data Science Jupyter Book, please use the "Cite this repository" button on the Toolbox repository's landing page in the right sidebar.
If you would like to cite an individual method's Jupyter Notebook, click on the "Notebook Repository" button at the top of the notebook and then use the "Cite this repository" button on that repository's landing page.
See here for more information on how GitHub generates these citation buttons from CITATION.cff files.