A huge aspect of The Dark Between by Sonia Gensler is the Spiritualist movement, which claimed popularity in the U.S. and Britain from the 1840s through the 1920s.
The movement is largely associated with the popularity of mediums, seances, and “spirit photography” during this time period — but as Gensler pointed out in one of her blog tour stops, Spiritualism “is considered by its followers as a religion.”
In general, Spiritualists believe in communicating with spirits and that the soul continues to exist after death. Sonia noted in her blog tour that the Spiritualist Church in Cambridge, England defines their beliefs as such:
All religions believe in life after death but only Spiritualism shows that it is true by demonstrating that communication with departed spirits can and does take place. Spiritualist Churches provide one of the venues where communication, through Mediumship, is possible and many loved relatives and friends take advantage of this opportunity to continue to take an interest in our welfare. [. . .] It is scientifically proven that matter (being part of the creative force, or energy) cannot be destroyed; it merely changes its form. Spirit, as part of the Creative Force is, therefore, indestructible. On the death of the physical body, the spirit continues as an integral part of a world, which interpenetrates our world but in a different dimension. This world is referred to as the Spirit World.
The Spiritualist movement also gave rise to so-called “Spirit Photography,” in which images of ghosts were supposedly captured on film.
The practice, of course, has been proven a hoax. Most spirit photographers used various tricks like double exposure to create so-called “spirit images.”
As Sonia wrote in another of her guest posts:The Spirit photography phenomenon started in 1861 when a Boston jewelry engraver, William Mumler, took what turned out to be a very unique self-portrait. Upon developing the plate, he saw the image of a woman next to him — someone he knew at once to be his long dead cousin. Prominent spiritualists immediately latched on to this as proof of the survival of spirit after bodily death. Spirit photography soon became all the rage, and many unscrupulous photographers committed fraud in their zeal to profit from its popularity.
One of Mumler’s most famous portraits is the one shown here, featuring Mary Todd Lincoln and the supposed ghost of her late husband, Abraham Lincoln.
Sonia shares these links to check out collections of spirit photographs:
Novel Novice 
This looks like an awesome book! Definitely going onto my tbr list! For students interested in Spiritualism, it would be great to read THE DARK BETWEEN alongside IN THE SHADOW OF BLACKBIRDS by Cat Winters, which also features spirit photography.