This adorable book is the first in the Letter Friends series by first-time author Lynette Teo. While A always gets to be first and Z has the final say, X and O are the quiet overachievers of the lot. X and O are B.F.F.s! is a warm and light-hearted tale about X and O—best friends, partners in crime, the yin and the yang. This story is an engaging read about how X and O together, have a role in the language of love, music and play..