A second, larger book of elegies was published perhaps a year later, one that includes poems addressed directly to his patron and (as expected) praises for Augustus. The 19th century classics scholar Karl Lachmann argued, based on the unusually large number of poems in this book and Propertius' mention of "tres libelli", that the single Book II actually comprises two separate books of poetry conflated in the manuscript tradition, an idea supported by the state of the manuscript tradition of "Book II." An editor of Propertius, Paul Fedeli, accepts this hypothesis, as does G.P. Goold, editor of the Loeb edition. The publication of a third book came sometime after 23 BC. Its content shows the poet beginning to move beyond simple love themes, as some poems (e.g. III.5) use "Amor" merely as a starting point for other topics. The book also shows the poet growing tired of the demanding yet fickle Cynthia, and implies a bitter end to their torrid love affair. Book IV, published sometime after 16 BC, displays more of the poet's ambitious agenda, and includes several aetiological poems explaining the origin of various Roman rites and landmarks. Book IV, the last Propertius wrote, has only half the number of poems as Book I. Given the change in direction apparent in his poetry, scholars assume only his death a short time after publication prevented him from further exploration; the collection may in fact have been published posthumously. It is also possible that Propertius had children, either with Cynthia or a later liaison. An elegy of Ovid dated to 2 BC makes it clear that Propertius was dead by this time. Propertius' fame rests on his four books of elegies, totaling around 92 poems (the exact number cannot be known as over the intervening years, scholars have divided and regrouped the poems, creating doubt as to the precise number). All his poems are written using the elegiac couplet, a form in vogue among the Roman social set during the late 1st century BC. Like the work of nearly all the elegists, Propertius' work is dominated by the figure of a single woman, one he refers to throughout his poetry by the pseudonym Cynthia. She is named in over half the elegies of the first book and appears indirectly in several others, right from the first word of the first poem in the "Monobiblos": Apuleius identifies her as a woman named "Hostia", and Propertius suggests she is a descendant of the Roman poet Hostius.