Being a practicing pagan for the past few years I've made a point in dabbling in as much as I can despite settling on the current mix that makes me an eclectic pagan or a heathen depending on your standards or definitions of those sections of the religion. No matter which pagan word you choose to call my belief system magick has been a big part what I have been doing for the last couple of years of my practice.
What many people do with their practice is to buy 1 or more Book(s) of Shadows from a magick/craft store they found on line early in their practice. For those of you who don't practice the trade a Book of Shadows is a book on Gods, Goddesses, beliefs/traditions, and spells. Personally I have never trusted any of these commercially made Book of Shadows.
The spells I really don't trust. The original purpose of the spell is often only known to the writer and they can change a single thing and claim that the spell does something different just because they changed something. This I don't trust because for all I know the spell (even now that it has been changed to do something else) will still do what it was originally intended for. I would rather write my own spell, that I know has not been altered in any way from the original, and cast it on my own using the exact ingredients and objects listed and have it do the exact thing that I intended.
The Gods and Goddesses I could buy an entire separate book on, if I'm too lazy or don't have the money to buy the book I can easily access one at a local library as long as my card is in good standing. This will also give me access to books by several different authors and their different opinions on the God or Goddess (or maybe even universal energy depending on what I decided to look up at the moment). It offers me more opinions than just those of the author of the Book of Shadows.
The beliefs/traditions I also have a slight (though not huge) issue with. Many people will follow the beliefs and traditions that are listed and/or talked about in that book but not all will. Some may follow the one specific belief system listed in the book while others may choose to mix those traditions with some others they are reading about. The traditions may also vary from person to person since we all may have different interpretations of what the texts mean. The Wiccan Rede (listed at http://www.witchway.net/wicca/rede.html), which is not followed by all Wiccans and pagans, can be widely interpreted by all pagans because of the way it is written. Each interpretation is up to the follower (or the coven if they don't choose solitary practice) and is a highly personal thing if they choose to follow the rede.
Personally I prefer to write my own Book of Shadows. It is specialized to my belief system, my Gods and Goddesses, my spells, and any traditions I choose to create for myself and for my way of worshiping those in whom I follow. It caters to whatever religious needs I may have and can be added to as many times as I choose too and can be lengthened to as many pages as I like.