Dr. Roland Stull, professor in the Department of Earth, Ocean and Atmospheric Sciences at the University of British Columbia's Vancouver campus and author of several widely-respected meteorology textbooks, has made the contents of his book Meteorology for Scientists and Engineers (3rd Edition) available to the public at no charge. Alongside this 2011 edition he is making available a slightly revised version, completed in 2015, retitled Practical Meteorology.
Both books have been released to use and share for free under a Creative Commons License. According to the books' web site, they cover the same topics in the same order, and share the same index.
From the Preface:
This is a book on practical meteorology. I wrote it for students and professionals in science and engineering who want to understand basic concepts, but who don't need to derive equations.
To make this book more accessible, I converted all the equations into algebra. With algebraic approximations to the atmosphere, you can see the physical meaning of each term, and you can plug in numbers to get usable answers.
No previous knowledge of meteorology is needed — I start from the basics. Your background should include basic algebra, trig, and classical physics. This book could serve the fields of Atmospheric Science, Meteorology, Environmental Science and Engineering, Air Quality Meteorology, Climatology, and Geography.
Both versions of the book are available to read online or download in PDF format from the books' web site.