Recently, I've come across Planck's lectures in theoretical physics and I have a couple questions. Although I'm curious about the series as a whole, I'm mostly interested in volume 1: General Mechanics and maybe Mechanics of Deformable Bodies. The Series consists of: General Mechanics, Mechanics of Deformable Bodies, E&M, Optics, Theory of Heat. I am not referring to the 8 Lectures book that is currently a Dover reprint. 1. Any one have experience with General Mechanics or any of the texts in the series? 2. Which books in the series would still be the most valid? 3. What level (upper undergrad, early graduate, etc.) would they be? 4. Anyone know if there is still a copyright on them or if the copyright has expired? I've looked for any info on the copyright status of these books and was unable to find anything. Just to make this clear, if the copyright is still valid and if I still decide to read them, I will be buying it used off of Amazon.