It depends if you are a medical student or a residet.
As a medical student, you'll find most of what you'll need (wards, shelf, etc) using Beckmann, and also Blue Prints.
As a resident, you'll need a couple of resources. One is your basic text book. This varies per region in the country. In the South and Southeast (generally, NOT always) Williams is favored considering it comes from UTSW and UAB. Out here and NE Gabbe is always a popular choice. You should go with one of them as your "basic" back up for reading about the pathophysiology, and the historical aspects of pre-eclampsia, GDM, etc. and on your Compendium (which you'll get from ACOG) as your "up to date" source for practice guidelines.
Lastly, almost all the residents in our program rely on UptoDate.com, as it is hands down a life saver, especially when you're faced with a tough case in the middle of the night.
Good Luck!