With the increased seismic activity in the Yellowstone Caldera, it is likely that there is some increased interest in in the geology of the area. Here are some resources that should be of interest.
First, we have a fairly recent peer reviewed publication on the "Super Volcano" known as Yellowstone, including some discussion of just what a "Super Volcano" is.
The largest scale of volcanic eruptions, the so-called super-eruptions, can
destroy all living beings and infrastructure over tens of thousands of square
kilometres, can disrupt agriculture over millions of square kilometres and can
alter global climate for years or decades. As such, society must endeavour to
create reliable volcano-monitoring systems that can detect the sorts of Earth
processes leading to large-scale explosive volcanism. Although the volcanological
community has had some success in predicting small eruptions, the scarcity of
great eruptions over the past 150 years means that we have little experience
understanding the prelude to major events. This is particularly true at caldera
systems, which are capable of large-scale volcanism and exhibit frequent unrest
but have undergone only small eruptions historically...
The paper has some excellent graphics and the discussion of the geology is quite accessible.
Figure 2. Diagram illustrating seismic-wave-velocity anomalies in the shallow crust beneath Yellowstone as viewed from the southwest (adapted from Husen et al. 2004a). The orange volume outlines the anomaly attributed to partially molten rock extending above the main magma chamber (and beneath the surface expressions of the Sour Creek and Mallard Lake resurgent domes). The red volume is an anomaly with properties suggesting gas-filled fractured rock. The green dots are hypocentres from the 1985 earthquake swarm. The arrows are postulated trends of hydrothermal fluid flow from the magma body to the inferred gas-filled body ...
The paper by Lowenstern, Smith and Hill is available here (full citation below)
But wait! Before you download that, you may be more interested in a more recent and more written for the general public piece by Lowenstern and Hurwitz, available here. It's got even better graphics and is even more accessible.
Schematic cross-section of the crust beneath the Yellowstone Caldera based on Hildreth (1981) and Husen et al.
And when you are done reading these papers, you'll be the most well informed non-geologist visiting the USGS Yellowstone Volcano Observatory web site, located here.
Lowenstern, J.B., Hurwitz, S. (2008). Monitoring a Supervolcano in Repose: Heat and Volatile Flux at the Yellowstone Caldera Elements, 4, 35-40
Jacob B. Lowenstern, Robert B. Smith, David P. Hill (2006). Monitoring super-volcanoes: geophysical and geochemical signals at Yellowstone and other large caldera systems Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, 364 (1845), 2055-2072 DOI: 10.1098/rsta.2006.1813