Helena (or Helen) was born about 255, and married to the Roman general Constantius Chlorus, who became emperor of Britain, Gaul, and Spain when Diocletian divided the Empire. In 274 she bore him a son, Constantine, but in 292 he divorced her in order to cement a political alliance by another marriage. Most historians say that she was born in Drepanum (now Helenopolis) in Asia Minor; but an old tradition asserts that she was born in Britain, in Colchester (51:54 N 0:54 E), and was the daughter of the chieftain Cole, remembered today as Old King Cole. If so, she may have been a Christian from birth, since Christianity was well established in that region. In 306, after the death of Constantius, the army at York (53:58 N 1:05 W) proclaimed Constantine emperor in his father's place, and by 312 he was master of the Western Empire and issued an Edict of Toleration that made the practice of Christianity legal for the first time in over 200 years. Helena worked enthusiastically to promote Christianity, and eventually went to the Holy Land, where she spent large sums on the relief of the poor and on building churches on sacred sites. She is particularly associated with the discovery at Jerusalem (31:47 N 35:13 E), near the probable site of Calvary, of a wooden cross that was accepted as the actual cross on which Jesus was crucified. (The earliest accounts of this discovery, however, do not mention her.)
At least two prominent English novelists have written about her. Evelyn Waugh wrote a novel called Helena, and Dorothy L Sayers wrote a play called The Emperor Constantine for the 2000th anniversary of the founding of the city of Colchester. I have not read the former, but can recommend the latter as interesting and informative, both historically and theologically.
Almighty God, who didst call thy servant Helena to an earthly Throne that she might advance thy heavenly kingdom, and didst endue her with zeal for thy Church and charity towards thy people; Grant unto us thy people that we may be fruitful in good works, and steadfast in our faith in thee, and finally by thy mercy may attain everlasting life; through Jesus Christ our Lord, who liveth and reigneth with thee and the Holy Ghost, one God, now and ever.
Almighty God, who called your servant Helena to an earthly Throne so that she might advance your heavenly kingdom, and filled her with zeal for your Church and love for your people; Grant to us your people that we may be fruitful in good works, and steadfast in our faith in you, and finally by your mercy may attain everlasting life; through Jesus Christ our Lord, who lives and reigns with you and the Holy Spirit, one God, now and ever.