I like how the thread devolved from recommendations on what to use on his tablet and PCs to "You should get e-ink device X, Y and Z."
Personally, for the brief stint I had an e-ink device handy, I still found myself reaching for my tablet because it is faster, sharper, has better contrast, and doesn't rely on ambient light. On Android, I'm a big fan of Mantano for epub and PDF. For Amazon books, I just buy and read them via Kindle for Android, rather than go through the separate downloading, stripping and converting to merge them with my library. Yes, I run the risk that if Amazon decided to kill my account, I would lose access to the books (well, in reality, only one book so far).
Calibre probably is the best cross-platform manager, though it's so complex it can cause a lot of headaches, too. It becomes worthwhile when your library passes about the 800 book mark and you want to organize everything using metadata and push different collections to different devices. Prior to that point, if you just want to convert a couple of files or drop this or that book on a device, it's like using a front-end loader to hang a painting.
A simpler option for DRM-free epubs (and a few other formats) on Windows and Linux is FBReader. It won't copy everything to a new location or rearrange your folders when you add books to its library, and it's a much better reader than Calibre, though if you have a tablet I doubt you'll actually do much reading on the PC or laptop.