II. Military options
These are all hard to sell without Iranian provocation. And Iran's history shows they don't do much of such at all. If they act they do covertly to have some plausible deniability - just like how the US likes to act if has no justification.
1. invasion
Going gung ho all in AMERRRICA, FUCK YEAH! style. Invade it like Iraq or Afghanistan. They say the only obstacle is the terrain. Iranian military is weak indeed, for all the anti-coup redundancy measures, see Luttwak. The book says just to dismantle the nuke program and such, not to reform the country how they tried in Iraq and Afghanistan. The US is great overthrowing regimes, but setting up working ones instead is a harder question.
High cost, high manpower needs which the US might not has. Very few would support this. Back then it was true and it is true right now. Even the airstrike got flak this year. Since the publication of the book they failed in Afghanistan. No appetite now.
Neighbors might not help either so limited way of entries.
2. airstrike
They mention two types. A "coercive" bombing campaign - like Rolling Thunder or Linebacker - to bend Iran to will, which is less feasible, and one that aims to disarm Iran, trying to destroy the nuclear program with bombings. The latter being in the realm of possibility.
They write about the difficulties and tasks to solve - but all within the capabilities of the US. Can be initiated any time and could take from a few days to several weeks, but they need intelligence on the targets which could take some time to gather. They need to dismantle or suppress air defense first, next they have to target the components of the program: the facilities, the researchers, ballistic missile program. Targets are numerous.
Scaled down and up version exists, depending on what they wish to achieve, the limited version would only target key facilities.
Back then in 2009 they estimated Iran will acquire Da Bomb sometimes in 2010-2015. They also estimated that if they did an air campaign against Iran it would delay them by 2-10 years - lower numbers being more realistic, and even 5-7 years would make everyone ecstatic. They had no illusions they can stop it once and for all.
The model was the Israeli strike against Syrian (Deir ez-Zor) and Iraqi (Tuwaitha/Osiraq) nuclear facilities, with the caveat that it wouldn't anything be like those.
They note such an attack might solidify Iran's resolve to acquire nukes, could spark the rally around the flag effect, Iran might strike back with covert tools, and make them more radical in general.
This policy can be repeated as many times as necessary.