I fiddled with Calibre a few times in the past, mostly when I had to convert an ebook into another format. I usually gave up on it afterwards, because I had my own way of organizing my ebooks, which clashed with Calibre in so many huge ways.
But now that I have a smartphone, of course I want to access my books from it. It took a bit of study and stress, but I managed to setup the smartphone to my wifi, and setup quite a few automated syncing services, and was pleased with myself with those accomplishments.
But "modern" readers are quite persnickety when it comes to how they'll list libraries, and they all link back to Calibre for managing libraries. So far, I have numerous library folders all over my hard drive from starting over many times because I keep trying to use my old system, which apparently is flawed and old-fashioned.
But since it seems modern readers require Calibre's way of organizing a library, I'll have to drag myself into the present and learn how to do it! Hopefully Overdrive will be similar once I tackle that system ^_^
Meanwhile, I found such an awesome program: STDU Viewer is a free viewer for multiple file formats: TIFF, PDF, DjVu, XPS, JBIG2, WWF, FB2, TXT, Comic Book Archive (CBR or CBZ), TCR, PalmDoc(PDB), MOBI, AZW, EPub, DCX and image (BMP, PCX, JPEG, GIF, PNG, WMF, EMF, PSD) files. Once I see that it handles all the files I've highlighted, I'll be able to uninstall so many programs, yay! And it's FREE for non-commercial use! PS: It rotates comics so we can turn our laptops, and has the option to rotate one, or all in the opened file - sweet!