Your situation:
My daughter was in the extension class in a public primary school. We started working on this book during the end-of-school-year holiday (after Grade 5 finished before Grade 6 started), preparing her for the selective high school entrance test that happens at the beginning of Grade 6.
Why you liked/didn't like the book:
We liked this book (we only used book 2). It indeed teaches reasoning skills, instead of just giving questions, as a normal workbook would do.
This books only have questions, it uses incremental questions to explain a math concept.
It covers many basic topics in primary maths. One or two pages (3 to 8 questions each page) are dedicated to one topic, sometimes three pages. But I wish there were more.
Book 2 is for Grade 5 and 6. Almost all of the questions seemed to be easy. But when I checked the teacher's manual, I found there was more to teach apart from just giving the answers. The teacher's manual is a must to use this book. The teacher's manual explains the key teaching point of the concept hidden in each the question.
We sat together and worked every problem out. We looked at the example problem together than she worked on the rest of the problems on the same page by herself. As she was working through the page I checked her answers against the teacher's book and corrected her mistakes straightaway before we moved on to the next page.
Overall we liked the book. It teaches more than just being a workbook. We just wish there were more questions under each topic.
Any other helpful hints:
This is not a independent workbook. The parent need to sit with the child working together on this book.
The teacher's book is very very helpful and I would have missed some key teaching points if I didn't have the teacher's book.
This book can be a supplementary to your normal math curriculum.