The Book of the Law/Liber Legis is a good place to start, however, it will be confusing. Instead I would look at Crowley's commentaries on this book which can be found online.
The link above is a good collection of documents concerning Thelemic principles in terms of a philosophy.
A great beginners book written by Crowley is Magick without Tears. After this, I would check out Liber ABA/Book 4. All of this can be found online for free.