May 22. On this date in 1984, the Universal House of Justice addressed a letter to the National Spiritual Assembly of Taiwan, advising them to "base your translations on current editions of all the books referred to, if translation is involved. In each instance you should consult the original publisher and obtain a copy of the latest printing or edition to ensure that all approved corrections are embodied in your translation." Many Bahá'í books have had significant edits, deletions, and additions since their authors' deaths for various reasons, including failed prophecies.

May 22. On this date in 1984, the Universal House of Justice addressed a letter to the National Spiritual Assembly of Taiwan, advising them to "base your translations on current editions of all the books referred to, if translation is involved. In each instance you should consult the original publisher and obtain a copy of the latest printing or edition to ensure that all approved corrections are embodied in your translation." Many Bahá'í books have had significant edits, deletions, and additions since their authors' deaths for various reasons, including failed prophecies.

371. Translators Should Utilize Most Recent Editions of Books
"The Universal House of Justice has requested us to advise you to base your translations on current editions of all the books referred to, if translation is involved. In each instance you should consult the original publisher and obtain a copy of the latest printing or edition to ensure that all approved corrections are embodied in your translation."
(From a letter written on behalf of the Universal House of Justice to the National Spiritual Assembly of Taiwan, May 22, 1984)

George Townshend was an Irish Anglican clergyman who had renounced his orders to the Anglican Church in 1947, at the age of 70. Shoghi Effendi designated him a Hand of the Cause of God in 1951 and once said of George Townshend that he feels "Mr. Townshend's services to the Faith can best be rendered by his writing about it, as he obviously has an outstanding ability in this direction..." Of the books that Townshend wrote, Christ and Bahá’u’lláh is notable for the changes made from the original publication to subsequent editions published after the passing of Shoghi Effendi. For example, a statement about the "first and present Guardian" has been removed and a section discussing "the lineage of succeeding Guardians" has been replaced with a section discussing "divinely guided institutions" in general.
John Esslemont's book Bahá'u'lláh and the New Era remains an important text that has been used in Bahá'í missionary activity. However, from in its initial publication to later editions, Bahá'u'lláh and the New Era has been significantly edited, with references to Avarih removed in subsequent editions published after Avarih's apostasy from the Bahá'í Faith.
Other significant edits include...

Perhaps the most important change in Bahá'u'lláh and the New Era was made on page 212 of the 1923 edition. Recorded as a Bahá'í prophecy (59) concerning the "Coming of the Kingdom of God," Esslemont cited Abdu'l-Bahá's interpretation of the last two verses of the Book of Daniel from the Bible. He stated that the 1335 days spoken of by Daniel represented 1335 solar years from Muhammad's flight to Medina in 622 A.D., which would equal 1957 A.D.. When asked "'What shall we see at the end of the 1335 days?'," Abdu'l-Bahá's reply was: "'Universal Peace will be firmly established, a Universal language promoted. Misunderstandings will pass away. The Bahá'í Cause will be promulgated in all parts and the oneness of mankind established. It will be most glorious!'" (60) In editions published after his death, Esslemont's words have been changed to say that Abdu'l-Bahá "reckoned the fulfillment of Daniel's prophecy from the date of the beginning of the Muhammadan era " (61) and one of Abdu'l-Bahá's Tablets is quoted on the same subject in which he writes, "'For according to this calculation a century will have elapsed from the dawn of the Sun of Truth....'" Esslemont appears to conclude that Abdu'l-Bahá was referring to the year 1963 and the one hundredth anniversary of Bahá'u'lláh's public claim to be a Manifestation of God. (62) These words, however, were never written by the author, but were added posthumously. And, it should be noted that the phrase "'the dawn of the Sun of Truth'" is not a reference to a particular year, in this case 1863, but to a period of years when the Bab and his followers were preparing the way for the Manifestation of Bahá'u'lláh. Hence, they are commonly referred to as the "Dawn-Breakers." (63) Further, in another quotation which originally appeared on the same page, but was also removed from later editions, Abdu'l-Bahá plainly stated, "' This is the Century of the Sun of Truth. This is the Century of the establishment of the Kingdom of God upon the earth.'" (64) Esslemont recorded Abdu'l-Bahá as declaring explicitly that the prophecy was to be computed from the Hijra or 622 A.D. and that specific conditions would exist in the world upon it's fulfillment in 1957. When it became apparent that this Bahá'í prophecy would not be fulfilled, it was replaced with the ambiguous material which has remained in the text to the present. This is evident from the fact that, although Esslemont's other eyewitness accounts were removed in the 1937 revision, the record of Abdu'l-Bahá's prophecy was left intact by the American National Spiritual Assembly and Shoghi Effendi. It was not changed until after 1957. (65) Also, Abdu'l-Bahá's conviction that all of these events would take place in this century have been expressed in other writings and it is evident that Shoghi Effendi shared his optimism as well. (66)

To summarize rules of Bahá'í translations...
Arabic works cannot be translated into Persian.
Persian works cannot be translated into Arabic.
Persian and Arabic works can be translated "into Oriental languages such as Turkish and Urdu."
Persian and Arabic works can be translated into English.
Persian and Arabic works cannot be translated into European languages.
English works can be translated into European languages.
For further reading, please see "Persian, Arabic, and Provisional Translations", "Translation and provisional translations", "Unveiling the Hidden Words, by Diana Malouf: Commentary on "Translating the Hidden Words,' review by Franklin Lewis", a letter dated June 27, 1933 from Shoghi Effendi to Adelbert Mühlschlegel about Bahá'í orthography, a letter dated June 30, 1999 from the Universal House of Justice outlining its procedures for approving provisional as opposed to authorized translations, a letter dated May 13, 1986 from the Universal House of Justice to the Spiritual Assembly of Guadeloupe, and a letter dated February 15, 1957 from Shoghi Effendi to French Bahá'ís.