The Polyglot Project, a collection of language learning tips from polyglots and language enthusiasts across the globe (including yours truly), is now available as both a physical book on Amazon and as a free PDF download. This tome of language awesomeness contains over 500 pages of language learning goodness from 43 authors!
Michael Heim (1943-2012) was a highly respected professor of Slavic languages at UCLA. He spoke 10 languages (though he claimed that the number is hard to tie down due to the slippery political nature of language-dialect distinctions). Heim believed that effective language learning must begin (and progress) with a specific purpose in mind; he never learned a language just for the heck of it.