Wikibooks is a great idea. Like Wikipedia, Wikibooks consists of content that is public domain, which means that anything you find there is reusable in any fashion you like. This means it is truly Free and Open for learning and teaching.

Currently the Polish Language Wikibook consists of a basic course for learning Polish covering pronunciation, nouns, genders, adjectives, question words and more.

As always with Wikimedia projects, you are free to improve on the information created by fellow humans!

From Website

The Polish language is a member of the Western Slavic group of the Indo-European family of languages. It is easiest to learn if one already knows some other related language.

The most closely related are other Western Slavic languages: Czech, Slovak, Kashubian and Sorbian. More distant are the Southern and Eastern Slavic languages like Russian, Ukrainian, and Serbo-Croatian. Even further, but still somewhat similar are the Baltic languages: Latvian and Lithuanian. Polish is spoken by a total of approximately 40 million people, making it the second most widely spoken Slavic language in the world, next to Russian, and whose influence is almost up to par with her bigger sister. Other Slavs, especially the Bulgars (who consider Polish the most popular third language there) understand Polish and can pick it up with not much effort.

Someone who doesn't speak any Slavic language, but speaks some other Indo-European language, may still find many similarities between Polish grammar and the grammar of that language as well as many similar words.

Learning Polish is very difficult for those who don't know any Indo-European language well, especially for those who speak only Chinese, Japanese, or Korean... but as long as you are committed to learning Polish it is possible. If you are familiar with any other Slavic language (i.e Russian), then you have an advantage as this group share very similiar grammatical structures as well as a common history among them from their rise as well-organised centralised kingdoms in the Middle Ages to the Communist era.
This Wikibook is designed for anyone who wants to learn the basics of the Polish language. It is suitable for beginners and those who've been learning the language for a few years. Polish is written with a particularly unique version of the Latin alphabet.

Visit the Polish Language Wikibook.