The gist of his argument is:
1. Conservatives elected Reagan, Gingrich, and GW Bush through "fusionism"--focusing the religious (social-conservative) and libertarian (aka small-government or economic-conservative) wings on a common "leave me alone" platform.
2. Dubya and Rove abandoned this in favor of using the gov't to win voters to the Republican party ("big-government conservatism").
3. Sager claims this is breaking up the "fusionist" coalition and dooming the Reps to electoral defeat, which looks prophetic now (though his gloating was restrained)
4. To keep the Reps in power libertarians should stop supporting big-gov conservatives and evangelicals should go back to trying to restrain the government instead of asking it to enforce their moral values.
It's a well done argument, though evangelicals can quickly note that the pitch to them boils down to "be reasonable--do it my way."
Personally I think he's barking up the wrong tree. In our system the biggest leverage goes to the swing voters, who decide the winner by being willing to go with either side. In 2000 that was people worried about how to pay for prescription drugs, so Bush and Gore made speeches about that issue while neglecting the concerns of the other 95% of the population.
Sager wants to make libertarians the core voters of the Republican party, displacing the evangelicals. That's a good way for a faction to get screwed. Sager documented how the evangelicals got only lip service from the Republicans they put into office. The same situation happens with blacks on the Democratic side of the aisle--lots of rhetoric, not much help with their real needs. The counter-argument is that core voters get a lot of leverage in primaries, but when core voters use that power they tend to produce candidates who can't win the general election.
Focusing on the Reps only would make sense if the Dems were a lost cause for libertarians--and there's a case to be made for that--but Sager documents the rise of small-government Democrats in the Mountain West region. If the Reps and Dems both have factions actively competing for the support of libertarians that's the best possible situation for us.
On the local level, the primary candidate I was volunteering for this spring was about as "fusionist" as Sager could have hoped for, with both factions supporting an insurgent against a pork-collecting incumbent. So a more useful book might be on how to find candidates who won't get assimilated by the Borg establishment once they get to the capital.