"had written" is best. The action is finished before or at the end of January, so it is correct to say "had written". But be prepared to hear the simple past "wrote" when talking with people. Here's an explanation: http://www.englishpage.com/verbpage/pastperfect.html

however, it is fine to use simple past in conversation and in American English ^__^

HAD WRITTEN

It depends on what you want the meaning to be. If you say "He had written 14 books...," that means that the action (writing 14 books) happened before another action that happened in the past. If you said "He had written 14 books by the end of last January," that would be perfectly acceptable. That would be the best choice.

"Had written" - I suggest that because the action is complete. "Wrote" is simple past, and isn't as definite nor clear as the Past Perfect "had written".