Cheresa At the end of chapter 22, Alessandra and Cristoforo are having a conversation about his connection to Lorenzo the magnificent. He mentions Lorenzo having a painting by Botticelli ( titled The Birth of Venus) and describes how the character Venus is portrayed in the painting. I'm thinking the characteristics of Venus are similar to Alessandra and may mirror her life which could be why the story is titled The birth of Venus.
Jess The Birth of Venus is one of Botticelli's most famous paintings and can be found in Florence today in the Uffizi Gallery. Venus is the goddess of love, the Roman equivalent of Aphrodite, so the title alludes to both the heroine's love of painting and of her coming of age - hope that helps! :)
This answer contains spoilers… (view spoiler)[In addition to what has already been said: 1) Venus was born of a violent act and Alessandra believed she was effected in the womb by her mother witnessing the bloody aftermath of the Pozzi Conspiracy, 2) Botticelli's painter was a great tribute to classical times but he would then become an adherent to the repressive Savonarola period which mirrors the evolution of Florence 3) finally, Alessandra does evolve into a women who finds (briefly) passionate love. (hide spoiler)]