Megilas Ta'anis is not Apocrypha, but rather a type of written Mishnah or Beraisa. Here is what we wrote on it in Shabbos 13b:

15) [line 22] MEGILAS TA'ANIS

(a) Megilas Ta'anis was originally composed by a Tana named Chananyah ben Chizkiyah ben Garon (along with students of Beis Shamai and Beis Hillel, according to the BEHAG, Hilchos Sofrim), approximately 100 years before the destruction of the Second Temple (see Shabbos 13b). It includes a list of the dates which were set aside as holidays, during the Second Temple period, due to the miraculous salvations and victories that occurred on them. On some of the dates, fasting was forbidden. On others, even delivering a eulogy was prohibited. Some salvations which occurred after the destruction of the Second Temple are recorded in Megilas Ta'anis as well (Rosh Hashanah 19a). These were added by later scholars and edited by Rebbi Yehudah ha'Nasi, the editor of the Mishnah (see Tosfos there DH Ha). It is apparent from the wording of the Megilas Ta'anis that we have that certain comments which appear in the Megilah regarding the cause for the celebrations were also added by a later editor.

(b) Megilas Ta'anis was committed to writing in order that these dates not be forgotten. For this reason it is referred to as a "Megilah" as opposed to the Mishnah and Beraisa which were still handed down by word of mouth at that time (Rashi, ibid.). It is written in Aramaic.

(c) According to some Tana'im and Amora'im, shortly after the destruction of the second Beis ha'Mikdash the holidays of Megilas Ta'anis ceased to be celebrated -- with the exception of the holidays of Chanukah and Purim (Rosh ha'Shanah 18b, 19b). The devastation that followed the destruction of the Second Temple served such a crushing blow to the Jewish People that it overshadowed all of the holidays of Megilas Ta'anis.

Best wishes,

Mordecai Kornfeld