One of my favorite newfound subjects. I'll give a few suggestions that are geared towards mathematicians and a few geared towards physicists.
Books geared towards mathematicians:
1)
https://www.amazon.com/dp/0817637079/?tag=pfamazon01-20 by Harold Edwards
This book is fantastic and is not written like just another textbook. He gives intuitive discussions of the material in the first three chapters and then goes on in chapters 4-6 to prove everything thoroughly. He also has a nice chapter on applications, which goes from complex analysis, the Lebesgue integral, and physics, even proving E=mc
2. Highly recommend for a first viewing of differential forms.
2) http://matrixeditions.com/UnifiedApproach4th.html" by John Hubbard
I will soon be getting this book. From what I've read of the excerpts, reviews, and table of contents, it looks to be a great book. Hubbard covers all the necessary linear algebra and presents to you calculus on manifolds, while integrating it into vector calculus. I look forward to going through this book. He also has some very nice physical applications, which includes Maxwell's equations.
3)
https://www.amazon.com/dp/0387480986/?tag=pfamazon01-20 by Loring Tu
The more abstract and general of the three books listed here, but it is still accessible to senior undergraduates. This book gives differential forms based upon their general definition, which requires the development of multi-linear and tensor algebra.
Books geared towards physicists:
1)
http://count.ucsc.edu/~rmont/papers/Burke_DivGradCurl.pdf" by William Burke
William Burke passed away young, so this book was unfinished by him. I've read there are a lot of mistakes, but it is well worth reading to get Burke's perspective.
2)
https://www.amazon.com/dp/0486661695/?tag=pfamazon01-20 by Harley Flanders
A nice amount of applications of differential forms written for physicists and engineers.
3)
https://www.amazon.com/dp/0486640396/?tag=pfamazon01-20 by Bishop and Goldberg
This is more rigorous than the two books above.
There is also
https://www.amazon.com/dp/0817644997/?tag=pfamazon01-20 by David Bachman. I didn't know which heading to fit it under. :) There is actually a thread here where someone wanted to get a group to go through the book and in which Bachman took part in, until mathwonk ran him off.