BibTeX:
@incollection{JEEsslemont1923,
  author = {J. E. Esslemont},
  title = {The Twin Holy Birthdays: Two Dawns, One Light},
  booktitle = {Bahá'u'lláh and the New Era},
  year = {1923},
  publisher = {George Allen & Unwin},
  url = {https://www.gutenberg.org/ebooks/19241},
}

APA:
J. E. Esslemont. (1923). The Twin Holy Birthdays: Two Dawns, One Light. In *Bahá'u'lláh and the New Era*. George Allen & Unwin.

MLA:
J. E. Esslemont. "The Twin Holy Birthdays: Two Dawns, One Light." *Bahá'u'lláh and the New Era*, George Allen & Unwin, 1923.

Chicago:
J. E. Esslemont. "The Twin Holy Birthdays: Two Dawns, One Light." In *Bahá'u'lláh and the New Era*, George Allen & Unwin, 1923.