I'm REAL excited about this question,but I don't have the time right now to think about it enough to post a list. I was actually going to compile one for Dover this summer-a long one. But I'll think about it and try and post a few at this thread.Here's a few to get started:
Elements of Homotopy Theory by George Whitehead:A classic by the master and it would be a fantastic resource for classical homotopy theory from a geometrical standpoint that can serve as a foundation for the modern,high tech treatment via model categories.Why it's out of print baffles me.
Analysis And Solution of Partial Differential Equations by Robert L.Street:There are so few good undergraduate textbooks on this subject and a nice inexpensive reissue of this book would go a long way towards assisting this situation.Wonderful discussion and lots of nice examples.
Notes on Differential Geometry by Noel J.Hicks: An absolute classic and it needs to be brought back for a new generation of graduate students-after being proofread carefully,of course.Graduate students learning differential geometry will wonder why people have been hiding it from them.
The Foundations of Geometry by K.Borsuk and Smilew:A lost classic on axiomatic treatment of the classical plane geometries from a modern standpoint.Another book that baffles me why it's out of print.
There-that'll get you guys started. I actually hope to post the full list at my blog this summer. I'll let you guys know when it's up for the world to see.