Update: The eSword links are all v8 files. Go to Biblesupport.com to search for v9 format files of these.
R.H. Charles’s massive two volumes on Old Testament writings: e-books (This is the R.H. Charles index page, because a number of his books are classics you might want to read)
Book of Enoch, Book of Jubilees, Sibylline Oracles: all for e-sword: here. (Lots more stuff to intrigue and confuse you here.)
A Miscellaneous Collection of Old and New Testament related writings in one big e-sword file: here (UPDATE: It occurs to me this is likely an eSword 8> file. Most people are using eSword 9< now. Try this link for what may be the same file (I don’t use 9+, and the description is nonexistent.)
You can find Targummim, Apostolic Fathers, and some Nag Hammadi writings, in original languages and translation at this excellent webpage: E-sword Original Languages Library (Under Resources: E-sword Modules on the left sidebar)
A great many of these texts were collated from the Internet, particularly the Internet Sacred Text Archive