I just passed, for the first time, and wanted to chime in, although others have responded already. The materials that prepared me the best were:

Breastfeeding Atlas
Core Curriculum
Breastfeeding and Human Lactation
Comprehensive Lactation Consultant Exam Review by Smith

If I had to chose between the Riordan book and the Core C. I would lean towards the latter. I found the Smith review to be invaluable, although it does have some mistakes in it (or rather glitches where the system picks the wrong answer). It has over 800 questions/answer and gets you in test mode and exposed to additional images. Same goes for the images in the Atlas.

I felt that much of the exam had to do with knowing how to take this type of exam, and therefore the sample tests where as important as the actual knowledge.

Since this was my first time and I didn't have a study group, I also joined a forum on e-learning, which gave me additional test exam exposure and the ability to ask questions. 

Hope that helps. Good luck with the learning!

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