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MOBI & ePUB will cover all current dedicated ereaders and reading software for at least one of these is available for all other current popular devices and computers with the possible exception of a few phones.
LIT and a number of older formats you don't list are necessary only to support older devices. DJVU, itself, is also a dying format though some of its characteristics are being integrated into the next major rev of ePUB.
PDF is useful primarily on larger display general purpose computers (e.g. desktop and laptop computers). Its not a good ebook format even there, except when dealing with books that require very strict formating and/or special fonts (e.g. technical textbooks, German texts in blackletter, parallel English & Greek texts, ...)
If you need to pare down your list of formats, I would place MOBI and ePUB as the primary keepers with PDF as a last resort format for those special docs that require it. Keeping TXT gives your users access to a format that can be accessed in any wordprocessor for editing or conversion.
While you're researching this, I would suggest that you look at other online libraries with an eye to what formats they offer. Project Gutenberg offers HTML, ePUB, MOBI, Plucker (why?, I don't know), QiOO, and TXT(UTF-8). Project Gutenberg Austrailia offers only TXT and HTML for most titles. The University of Adelaide's ebooks@Adelaide offers only HTML (formatted for web display only) and ePUB (for download).
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