Texts witnessed only in the Septuagint
[Edited by Ellopos from Wikipedia articles.]
The Letter of Aristeas or Letter to Philocrates [full text in the Greek original and in English is included here] is a Hellenistic work of the 2nd century BCE, assigned by Biblical scholars to the Pseudepigrapha.
Josephus who paraphrases about two-fifths of the letter, ascribes it to Aristeas and to have been written to a certain Philocrates, describing the Septuagint translation. Though some have argued that its story of the creation of the Greek translation of the Hebrew Bible is fictitious, it is the earliest text to mention the Library of Alexandria.
The Letter of Aristeas, called so because it was a letter addressed from Aristeas to his brother Philocrates, deals primarily with the reason the Greek translation of the Hebrew Law, also called the Septuagint, was created, as well as the people and processes involved. The letter’s author alleges to be a courtier of Ptolemy II Philadelphus (reigned 281-246 BCE).
Over twenty Greek manuscript copies of the letter are known to survive, dating from the 11th to the 15th century. The letter is also mentioned and quoted in other ancient texts, most notably in Antiquities of the Jews by Josephus (c. 93 AD), in Life of Moses by Philo of Alexandria (c. 15 AD), and in an excerpt from Aristobulus of Paneas (c. 160 BC) preserved in Praeparatio evangelica by Eusebius.
In detail, the work relates how the king of Egypt, presumably Ptolemy II Philadelphus, is urged by his chief librarian Demetrios of Phaleron to have the Hebrew Law translated into Greek, and so add the knowledge of the Hebrews to the vast collection of books the empire had already collected. The king responds favorably, including giving freedom to Jews who had been taken into captivity by his predecessors, and sending lavish gifts (which are described in great detail) to the Temple in Jerusalem along with his envoys. The high priest chooses exactly six men from each of the twelve tribes, giving 72 in all; he gives a long sermon in praise of the Law.