Sowing, reaping, and garnering (harvesting) are terms with Biblical connotations that emphasize the results of Gradgrind's "sowing" a utilitarian philosophy in his children. The novel illustrates, attacks, and condemns the mechanization of life under industrialism and the way it warps the human spirit in service of greed and profit.
Gradgrind reaps and garners what he sows when he raises his two children, Louisa and Tom, to focus on the practical and material aspects of life to the exclusion of attention to the full human spirit, including the imagination and developing generous, loving hearts that can transcend financial calculation. As a result, Louisa ends up in an unfulfilling marriage with an older man and is tempted to elope with Harthouse. Tom becomes selfish and hardhearted and resorts to robbery to solve his problems. In both cases, Gradgrind realizes too late that the harvest of his educational methods is misery in one child and lack of moral decency in the other. The Gradgrinds garner lovelessness from the seeds of their philosophy—and that, for Dickens, is perhaps the saddest state of all.
This is a great question because it draws attention to the way that the very structure of the novel into three separate books helps to create and develop one of the central conflicts in the novel - the conflict between natural time (characterised in the passing of the seasons) and man-made time.
Consider how in Coketown and in Gradgrind's house, time is severely mechanised and controlled. Time is presented as something that is monotonous, relentless and regular, seen as an extension of man's desire to control life. Consider how Gradgrind has a "deadly statistical clock" in his office. Yet, as you have noted, the natural passing of time as summarised by the seasons and the processes of planting and harvesting are referred to through the naming of the separate books. Likewise, the narrator notes that even in Coketown the seasons still change. Dickens seems to build and develop this conflict to reinforce his central theme of the impact of the mechanisation of human existence. The variety of the seasons result in different landscapes and different jobs in agricultural terms, but in Coketown, industrialisation has replaced this with incessant, regular, predictable and unchanging toil.