How could critics read the construction of white identity? Whit identity has been built or based on what?

I think you're reading too much into this. This novel isn't concerned with the construction of "white identity." It's purpose was to assist the abolition movement and increase awareness amongst the middle class white Americans of the treatment of black slaves, particularly the women. It isn't about white women, or the identity of white women. To try and broach that subject in an essay would be like stirring up a hornet's nest. White women..... they've historically had thousands of different identities. Those identities have been based, on class structures, socio-econonic standing, parentage, and on and on and on........ If you want this paper to have an ending point, I wouldn't go in this direction. That was never the intention of this novel's author.

ok, but if we talk about the identity of blacks, then we should compare it with the identity of whites as the authors conceptualize. Do you know any book or article talk about how white identity constructed through dehumanization of blacks?

In the 1860's, only 1.4 % of the population owned slaves (United States). It you're writing about "true" history, you need to move away from novels that only portray "bad, threatening, cruel" slaveowners. If you're writing about the above novel, you already know how it portrays the owners and the slaves it speaks of. White identity cannot be construed or analyzed based on less than 2% of the American population.