TITLE: Texts on Principal Bundles, Characteristic Classes, Intro to 4-manifolds / Gauge Theory
QUESTION [16 upvotes]: I am looking for a textbook that might serve as an introduction to principal bundles, curvature forms and characteristic classes, and perhaps towards 4-manifolds and gauge theory.
Currently, the only books I know of in this regard are:

"From Calculus to Cohomology" (Madsen, Tornehave)
"Geometry of Differential Forms" (Morita)
"Differential Forms in Algebraic Topology" (Bott, Tu)

I have been reading both "Calculus to Cohomology" and "Geometry of Differential Forms," but am occasionally frustrated by the lack of thoroughness.  Both are at the perfect level for me, and cover almost exactly what I'm looking for, but I really prefer textbooks which are as thorough as possible, ideally to the extent of, say, John Lee's books (which I adore).  Meanwhile, Bott and Tu is a little advanced for me right now.
Of course, I don't mean to be picky, but I also can't believe that the three I've listed are the most thorough accounts of the subject.

REPLY [3 votes]: There are, by now, many excellent sources. A few off the top of my head:

Naber (2 volumes), mentioned above
Tu, Differential Geometry: Connections, Curvature, and Characteristic Classes
Nakahara, Geometry, Topology and Physics
Choquet-Bruhat, DeWitt-Morette, Analysis, Manifolds and Physics (2 volumes)
Bleecker, Gauge Theory and Variational Principles
Marathe, Topics in Physical Mathematics
Sontz, Principal Bundles: The Classical Case
Greub, Connections, Curvature and Cohomology (3 volumes)

For a high level overview see also the survey article Gravitation, Gauge Theories and Differential Geometry. I have quite a few more, but they may not be mathematical enough for your tastes. If you'd like them, just let me know.