I would recommend just getting the textbook and skipping the activity book and teacher's guide. I have both the textbook and activity book. The textbook is not really a "text"book but consists of 10 sets of worksheets covering the following topics:
Unit 1: Match and Sort
Unit 2: Numbers to 5
Unit 3: Numbers to 10
Unit 4: Order
Unit 5: Shapes
Unit 6: Patterns
Unit 7: Length and Size
Unit 8: Weight
Unit 9: Capacity
Unit 10: Compare Sets
Each unit has 20-25 pages of worksheets so there is really no need for a separate activity book. On their website, it says the activity book is for remediation and enrichment, but I found that the textbook already covered each topic very thoroughly - so thoroughly, in fact, that I often find myself skipping a few pages towards the end of each unit so as not to frustrate or bore Ella. Once she "gets" a certain concept, she has a very very low tolerance for repeating material. I think what I'll do instead is, after we finish the entire textbook, I will just go over the few pages in each unit that we skipped as a sort of review before proceeding to the next book. At the bottom of each page of worksheet, there is a pretty detailed instruction/explanation to the teacher or parent so I don't think a teacher's guide is necessary either. I certainly don't miss it.
Hope this helps!