ISIS has a lot of money. But we don't know for sure how much or where it comes from. Which is why a very rare leak of ISIS's internal books, published Monday on the blog Jihadology, is so interesting. One leading terrorism researcher says it's "possibly the best available primary source" on how ISIS funds itself — and it helps explain both some of ISIS's strengths and its fundamental weaknesses.

According to researcher Aymenn Jawad al-Tamimi, who posted the documents, they come from Deir ez-Zor province in eastern Syria. ISIS has firmly controlled the bulk of the territory there since July 2014, which means it's been able to set up a quasi-government — making the province at least theoretically a possible model of wider ISIS governance. The leaked figures cover the period of December 22, 2014, to January 22, 2015.

In Deir ez-Zor, during that time period, only a limited portion of ISIS revenues (27.7 percent) came from oil sales. That's especially interesting, according to al-Tamimi, because Deir ez-Zor contains the "best oil holdings in Syria."

Instead, the overwhelming bulk of the revenues, a little more than two-thirds, as recorded in the document and categorized by al-Tamimi comes from taxes (23.7 percent) and "confiscations" (44.7 percent) — outright seizing money and property from people living in ISIS's territory. Activist Iyad el-Baghdadi puts the conclusion pretty simply: "More than two-thirds of [ISIS] income is from extortion."

ISIS's reliance on extorting the people under its control isn't sustainable. That's not just because it is likely alienating those populations and exacerbating the risk of any uprising. It's also because you can only tax and steal from Syrians and Iraqis for so long before there's nothing more to take.

In a more limited sense, though, this is also a strength for ISIS. The US can bomb oil wells, but it can't bomb extortion networks. As al-Tamimi puts it, "The most vital IS revenues depend on the continued existence of its bureaucratic structure within the territories it controls, and there is little one can do to disrupt that short of destroying that structure militarily."

Al-Tamimi also examined ISIS spending during the one-month period in Deir ez-Zor and found something interesting. Almost all of ISIS's spending goes to its security apparatus — soldiers' salary, police, and military bases.

Actual governments spend much more of their budget on economic development and social services. It's not a shocking revelation that ISIS is bad at being a state, but this matters for the group's ability to maintain control. ISIS, then, can't keep up this funding model forever.

"It is theft that is filling IS coffers, not any kind of functioning economy," reporters at Germany's Die Zeit concluded after a separate, weeks-long investigation into ISIS funding. "The caliphate is unsustainable."