Through the Internet directly on the device. Depending on what operating system is installed on the tablet, download books through "PlayMarket" (for Android devices), the "AppStore" (for iOS users). In any of these book stores are mainly for a modest fee. Free ebooks online also can download. For example, owners of Apple products there is a special application "iBooks" where you can find almost any book for free. Users with Android gadgets can use the free apps in the "Game". But the choice of books is severely limited, because not much was given in the format of the application.
From computer to tablet. In this case you will need a special program to read on the device. For example, "FBReader" or "CoolReader". These readings support multiple formats: fb2, txt, epub, html, doc, rtf, pdb. It is possible to download the book in any format on the computer, and then transfer it to the tablet (e.g., via Bluetooth or by syncing via usb cable). On your tablet then you just need to open the desired file user-friendly application.
Through the browser on your tablet. If the device connects to the Internet (and almost all tablets, with very few exceptions, it is capable of) then just go to the browser in any digital library and download the book there for free. The most popular is the format "fb2" because the books are in this format occupy less space on the device and open any application to read. The format of the books, "doc" opens not only in reading but also various programs to work with documents. For Example, "MaxOffice". Through this app you can read books in doc, docx, txt, rtf.
Through a special application. If Apple products have the app "iBooks", where books can not only read, but save to device, Android devices have other useful applications. For example, the different online applications with the ability to read off-line. That is, you do not need to download the book itself. Just download the app, find the right literature to keep in the "read offline". After disconnecting from the Internet the book will remain available for reading at any time. To look for such applications it is expedient in the categories of "books and references".