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I recently discovered the charming treatise Non-Euclidean Geometry (1901) by Henry Parker Manning at Project Gutenberg. The book should be accessible to anyone with a high school course in geometry. The book is one of the more gentle introductions to non-Euclidean; however, by modern textbook standards it is somewhat dated.

I Googled Henry Parker Manning and discovered his is the first English language text on non-Euclidean geometry. You can find a short biography of Manning at this Brown University site.