I guess if you want to learn thinking but not necessarily math "thinking mathematically" above mentioned is your friend.
Each chapter includes 80-100 problems, divided in to usually between 4-8 sections. The problems are great, but once a section is complete they're weak on later refreshes. I've been working around this by doing even-numbered problems the first time through a section, then half of the odds a few days later when we're a couple of sections downstream, then selecting randomly from all of the unfinished problems in the entire curriculum for just a couple of extra "old stuff" problems each day throughout the year. We also supplement with a number of other great resources, if you're looking to implement a more problem- and exploration-oriented math curriculum:
1. Kitchen Table Math is great for selecting concepts to lead number talks with (for building number sense - this is the first part of our day)
2. Saxon has excellent spaced-repetition exercises for shoring up the calculation side of things, and giving the student some easy wins for confidence building (we typically use Saxon's material as a warmup before Beast Academy)
3. Thinking Mathematically (the one by J. Mason and L. Burton) has a unique and useful mental process for attacking hard problems when you're not handed a nice formula to plug things into. Once a week, we work through a hard problem using the method in this book.
4. I haven't worked it in yet, but Arthur Benjamin's "Secrets of Mental Math" has a lot of stuff in it that will solidly connect arithmetic and algebraic thinking later.
Two good books on this:
How to Solve It
http://www.amazon.com/How-Solve-It-Mathematical-Princeton/dp...
Thinking Mathematically
http://www.amazon.com/Thinking-Mathematically-J-Mason/dp/020...
And if you want to get better at sitting maths-based exams, here's my own book, Exam Mastery: How to excel in maths-heavy exams :
http://www.amazon.com/Exam-Mastery-excel-maths-heavy-ebook/d...
(No need to buy though - if you want it, shoot me an email and I'll just send it over to you.)