The Treasures of Darkness, Jacobsen's magnum opus, is a literary tour de force of history of Mesopotamian religion. The author analyzes its hitorical connotations and changes throughout the millennia (4th-2nd), all in the most exemplary academic manner.
According to Jacobsen, basic to all religion is a unique experience of confrontation with a power not of this world, which Rudolph Otto called "Numinous", that is, a confrontation with a "Wholly Other" outside of normal experience and indescribable in its terms; at best, it may be possible to evoke the human psychological reaction to the experience by means of analogy, which may, in turn, serve as ideograms or metaphors for it. Thus, this confrontation is terrifying, ranging from sheer demonic dread through awe to sublime majesty; it's also fascinating, with irresistible attraction, demanding unconditional allegiance. At its very core, then, the power of religion can possibly be derived from man's choice to sacrifice their volition at the altar of ephemeral promises and dubious safety, since they're no longer responsible for their actions as they have surrendered to the ultimate instance.
Mesopotamians experienced the Numinous as immanent in some specific feature of the confrontation, rather than as all transcendent. They saw numinous power as a revelation of indwelling spirit; that power was at the center of something that caused it to be, thrive and flourish. In contrast, Christian religion is wholly transcendent in terms of the Numinous, as the supreme deity is completely distinct from its tools, e.g. the burning bush of the Old Testament is only a manifestation of the power, not the power in itself. Whereas the power speaking to Moses in the desert detaches itself from the bush, which only served its situational purposes, numinous power in Mesopotamian religion didn't detach itself from the locus; it was the nature incarnate.
What's striking about Mesopotamian deities is they didn't reach out beyond the specific situation or phenomenon. They made no demands, didn't act, merely came into existence, were, and ceased to exist, and with them their characteristic phenomena. This process (phenomenon), called intrasitiveness, can best be illustrated with an example of a deity Dumuzi, the power of fertility and new life in the spring. There was no instance in which the god acted, ordered, or demanded; he just was or wasn't. He came into being in the spring, was celebrated as a bridegroom in the cult rite of the sacred marriage, was killed by powers of the netherworld, and was lamented and searched for by his mother and young widow. The conclusion is, the Ancient Mesopotamians regarded their deities in purely functional terms of a means to a desired end, as each deity had to serve its purpose; when it did, it was no longer needed until it was.
To bring about god's presence, in the rite of cult dramas, man would literally re-present the god by acting him and presenting his external form, to, as a consequence, become the god, who would perform the acts that fulfill the divine will with all its beneficent results for the community. Other means to re-present gods included cult images, reliefs, wall paintings, poetry, and building temples.
As has already been mentioned, there was in the experience of the Numinous both dread and fascination, and dread seemed to predominate. Even the word god in Akkadian (ilu) is associated with the feeling of paralyzing fear. Jacobsen quotes Gilgamesh from the Gilgamesh Epic:
My friend, you did not call me,
why, then, am I awake?
You did not touch me, why am I startled?
No god went by, why are my muscles paralyzed?
The Treasures... also traces the subsequent changes in perception of religion by Mesopotamians: the trend to humanization as the extension from the narrow pattern of family, occupation, and individual life cycle, to the wider patterns of the community and the state with its internal and external political forms; from anthropomorphism and sociomorphism to politicomorphism. Another great change overturned gods as rulers into more personally-inclined deities.
[All in all, the book can be recommended to anyone interested in history, and the history of Mesopotamia in particular, but also to people with interest in theories of religion.]