The great thing about having so many encyclopedias is that you can try a bunch of different things. If you have the time, altered books could be a lot of fun; if you journal at all, altering a whole encyclopedia could be a really cool way to express yourself. (Check out the <a href="http://embodiment.livejournal.com
" target=blank>embodiment</a> community and look at the picture posts for inspiration. Altered books into journals come out great.)
You could convert some into large blank books or sketchbooks, and then use the pages you remove for decoupage, collages, etc.
There are also artists who use old books to make art. I saw a book on this at Borders yesterday. They use the book and all its pages as a canvas.