All religions,over time, vary in beliefs,rituals,etc. None more so than Christianity! Just think of all those "churches" that are so different from the original. (Catholic). Even Judaism has variants, Russian Orthodox,Greek ditto. Two different "branches" of Islam!
Wicca is no different. I have been on this site for about ten years, and it seems that there are more "branches" of Wicca than ever; and more added every year!
Just as "Christians" interpret the Bible to suit their individual beliefs; so do Wiccans. The nearest Wiccan to the original I have met on this site is Lark; and even Lark will admit that she leans more towards Alexandrian Wicca than Gardnerian Wicca! (They are practically the same,anyway!)
All the books written since Gardner have been variants o the original;and that includes the best; The Farrar's, Cunningham, even Valiente, and others.
So, if you are a youngster wanting to learn Wicca, read first of the originals; the writings of Gerald Gardner and Alex Sanders; even some of Crowley's esoteric writings.
I grew up in England while these people were still alive. By 1954 evrybody in England, and beyond, knew of them. (Maxine Sanders is still alive!)
My advice to "newbies" is, if you want to study Wicca, don't pick the apples that have fallen from the tree! Go to the tree!
I am very sure that if Gerald Gardner was alive today he would not recognise the religion he started!