Amazon.com has a huge list of online books for the Kindle. If you don't have a Kindle, you can download an app for a PC or phone to read Kindle books. I'm still working my way down the list of Kindle Popular Classics(more than 16,000 free books and I'm only at 3000 or so), but I've found several by Robert Ingersoll and a few other atheist writers.
You can also do an Interlibrary Loan for any books that you want that your local libary does not have. Some libraries do offer e-books, I'm not sure how an inter-library loan would work for an e-book, but it may be possible. Good luck and if you find anything good, please let us know.
Oh and many of the books on Amazon have the "Look Inside This Book" feature, so you can read the first chapter of many of the good books.