I downloaded and re-installed Adobe Digital Editions so that I could get books from the library and read them on my tablet. It is not working and my tablet cannot open the books??

What can I do? I am 68 y/o and not very computer literate.  I put an micro SD card in  my computer to copy the books (also some ebooks that I bought from a retailer) onto the SD card so that I could read it on my computer or my ereader by puting the card into the reader (tablet or whatever you want to call it).. HELP