Finished last week reading the aforementiond book, as part of my research for my dissertation. It's my first Sombart, one of the fathers of Sociology, and I've found his style amusing, although somewhat lacking in rigor (he tends to provide some statistics and breakdowns of numbers -expenditure of some household, composition of the capital of some enterprise... without context, so it si difficult to assess how they fit in a bigger picture or how selective he has been in providing the data that supports his theses, omitting other pieces of information).
His main contention is twofold: Capitalism (or rather, the modern Western way of social organization) is a product of "luxury", understood as the production of highly valuable objects directed to the richest part of the population (what we call today "the 1%"), and luxury itself is a consequence of the "secularization of love", or the influence in tastes and patterns of consumption imposed by mistresses, thanks to their increased significance and social acceptance in the new urban environments enabled by the Renaissance (after the end of the Middle Ages).
The first contention is, in my humble opinion, not that off the mark, but probably not for the reasons Sombart presents. A developed luxury market is essential for the development of a capitalist society not from the producers perspective (because it requires high levels of capital and rationalization, not least because of the delays and uncertainties about payment from the dissolute nobility that initially comprises a significant portion of their clientele), but from the consumer's: sumptuary goods are required for the "keeping up with the Joneses" mentality that has kept pushing everybody in the West (and now in the rest of the world) to work more in the market (where the sale of their time was rewarded with the means of acquiring material goods, as opposed to housework, study, familiy, and of course quaint relics of bygone times as prayer and contemplation).
As for the second (the importance of women, specially illegitimate partners, in the birth of that luxury markets), I found it much less convincing. The urban development of the XV Century is indeed a watershed, as to kickstart the competitive showoffs between the different citizens, regardless of class, it is first necessary that each should have notice of the consumption level of his/her neighbors, but I don't think women played such a determinant part. Indeed, the enhanced recognition of misstresses is but an additional consequence of increasing inequalities in wealth: all societies where a few males have commanded a disproportionate percentage of the total resources tend to gravitate towards polygamy, be it simultaneous or sequential, normally with the more or less open complicity of females (in any assortative mating order any female on the bottom half of the desirability hierarchy is better off engaged with a fraction of a male in the top half than with her corresponding mate in the bottom half...)
However, an interesting read with a few tasty morsels to think about more leisurly, clocking in at less than 200 pages is a good investment, and left me with the desire to attempr the heftier Modern Capitalism (have to find a decent English translation first)