Walkers book about Gnosticism from ´83 is a decent work as an introductory to the subject, that also shows it's age and tendensies.
First off all, multiple things and statemens in this books is just false, which means you have to read this book with a lifted eyebrow on all pages. You can really feel that Walkers own tendensies comes out as almost disgust for the gnostics, which feels really out of place in an academic book. However, this might also be a production of the translation from Sphinx in Danish. This translation is packaged with The Gospel of Thomas in danish which is nice. It's, however, translated from english to danish, which is a big shame.
Walkers does give a really nice litterature list, and does reference a lot of words and passages in the book. It does feel a little fleeting when he chooses to cite, and when he dosen't.
All in all, this is a decent first step in learning about gnosticism, or getting a very very broad overview. It's an awfull book, if this is your first and last book on the subject though.