Ronsard’s love sonnets fall into two main groups, from opposite ends of his career. The early Amours have sonnets by their hundreds represent Ronsard in his early, experimental, enthusiastic phase, initially full of obscure learning and complex neologisms, then simpler and more direct. The late Sonnets for Hélène represent the mature, cooler Ronsard but are also more carefully crafted and (relatively) fewer in number – though there are as many just for Hélène as Shakespeare wrote altogether.
The Amours recount two of Ronsard’s love affairs: the first with Cassandre, the second with Marie. Cassandre is said to be Cassandre Salviati (1530-1607), daughter of an Italian banker, whom Ronsard met at a court ball on 21st April 1545, when he was 20 and she was 15. As a man of the church, Ronsard was unable to marry; but it clearly didn’t stop him falling in love…! She features in the Odes, published in 1550, before getting her own volume, book 1 of the Amours, in 1552. The Cassandre poems are almost all in decasyllabic lines, show very little variation in metrical form, and in them Ronsard is keen to develop French poetry along classical lines with plenty of mythological allusions and a ‘high style’.
Marie was youngest daughter of Étienne Guyet, a minor noble; though she did not return Ronsard’s love and married another, her early death in the early 1570s brought a wonderful series of sonnets from Ronsard, more sombre in tone than the first part of the book but balancing it in their elegance. This volume came out in 1556; its continuation in 1574. The Marie poems are more heterogeneous than the Cassandre set, and in fact the collection as represented in the oeuvres complètes was built up from several independently-published sets: in these poems Ronsard has developed his view of French poetry, and now favours the alexandrine (12-syllable line) as being ‘more French’, as well as introducing more colloquial, day-to-day vocabulary and more exclamations, terms of endearment etc into the poetry in place of the earlier ‘high style’.
In the 1570s Ronsard returned to the theme of love and the form of the sonnet, with two books of Sonnets for Hélène. By contrast with the earlier love affairs, this one with Hélène is, as far as we know, a ‘courtly’ love although Hélène herself was real. It is, however, open to debate how much any of these poetic collections reflect real love affairs: whether Ronsard was in love or not at an early stage with any of these ladies, the poetry was written over a longer period than the affair and is more laden with the traditions and effects of love poetry than the genuine experience of love.
As well as these books, Ronsard also published the “Amours Diverses“, as well as several other collections of Amours; and indeed moved poems from one set to another. There is evidence that Hélène, at least, complained about the fact that several dozen of ‘her’ poems had been previously published addressed to someone else! (Ronsard’s response was that she didn’t understand poetry, and the way that poets ‘have no regard to order and time – those are matters for historiographers’…)
On top of all this, Ronsard also made many changes, additions and subtractions in the various editions published in his lifetime – which included 6 versions of the ‘complete works’. Many of these have conventionally been collected into the “Recueil des Pièces retranchées des Amours” (‘Collection of pieces withdrawn from the Loves’).
Poems from all the selections of Amours are here:
book 1: Les Amours de Cassandre
book 2: Les Amours de Marie
Sur la mort de Marie
Sonnets pour Hélène
Les Amours Diverses
Recueil des Pièces retranchées des Amours
Oeuvres de Ronsard 
Very informative.