In "What's in a Name", the English translation of Zusha Wilhelm's sefer "Ziv HaShemot", the following is stated (Hebrew version with footnotes here):
1) Some say that one may name a male child after a female. (See Bris Avos 8:37; See also Koreis HaBris, Posach Eliyahu, note 8; See Kuntres HaShemos (revised edition), Vol 7, p. 10; See Sefer HaBris, p. 313; See the Midrash on Pinchas (13:12) ) Others maintain that it is better not to convert a feminine name to a masculine one. (See Sefer HaMetzaref chapter 3; Responsa Tzitz Eliezer, Vol. 11, ch. 56, and references to Vol 7, 49:13)
2) It is not proper to name a female after a male. (See Bris Avos 8:37) Some authorities disagree, and do have a custom of giving a man’s names to a female. (See Responsa HaMaor, Vol 1, p. 134, and in Responsa Mevaser Tov, stating that this is the custom in America. See Noam, Vol. 8, p, 192; See Devash , p. 299)
In the next chapter, the following is stated (Hebrew version with footnotes here):
Some say that one should not give his child a name that is common to both males and females. (See Responsa Divrei Malkiel 3:75; See also Responsa of R. Shmuel di Medina, Even HaEzer 65)
I got a hold of the printed book and added the sources to this answer. The book itself not only brings the sources, but quotes most of them.
As @ba pointed out in the footnotes, the Tzitz Eliezer 21:4:4 brings opinions that permit it, as well as opinions that are against it.