Reddit mentions of 5 lb. Book of GRE Practice Problems: 1,800+ Practice Problems in Book and Online (Manhattan Prep 5 lb Series)

Smart move taking the diagnostic first. Here's what I recommend you do in a step-by-step fashion:

• Go through the ETS Math Review Guide. Make sure you know ALL of the concepts in here and complete all of the exercises, even if they seem easy. The only thing you can probably skip is the "box plot" section.
  
  
• Go through your diagnostic test with a fine-tooth comb and try to determine precisely what your weaknesses are. You can break down math questions in four general categories: arithmetic, algebra, geometry, and data. Try to determine which of the four your strong at and which of the four you're not so good at.
  
  
• Obtain all of the official practice material, most of which you can find in the Super Power Pack. I would also recommend that you obtain the Old GREs because, although the math is a little bit easier, it has a ton of practice material (27 tests) that you can draw from.
  
  
• Purchase the 5-lb Manhattan book for supplemental math practice. Just make sure you don't do the verbal in this book.
  
  
• Start familiarizing yourself with some of the more common math strategies like Choosing Numbers and Periodically Stopping.
  
  
• Create a practice test schedule. When will you take your next practice test? When will you take the one after that? We use practice tests to gauge progress, so make sure you have a firm plan in place and that you take the practice test sessions carefully.
  
  
• Use this subreddit as a resource. If you ever feel overwhelmed, just give us a holler.