Thanks for sharing your tips. This could be very useful to RoboHelp users.
I assume the pages you referring is part of the topic.
Just to add some insights, TOC or Book with link to Topic inherits tagging property of Topics.
That is making Filter A-D work fine. As 1-3 was tagged part of topic and TOC cannot inherit that(as not very explicit reason) hence filter 1-3 does not work.
Other solution could be to tag TOC items, tag BOOK item with 1-3 additionally and tag TOC items pointing to page with individual tag.
As tagging topics with additional tag may give wrong result on Topic content or search filter.
Thank you for that insight. I now understand why a book with no linked topic worked (book inherits tags from its pages), and why the book with a linked topic did not (book inherits tags from its linked topic only).
So to sum up, the solutions are now :
1. If the book is linked to a topic, tag that book in the TOC with all the tags used in the topics that the book contains. Thus, I tagged my "all about" books with 1,2 and 3 in addition to their letter tag.
2. Don't link the books to topics. Instead, just use that topic as the first page in the book (as we always had to do in the past). This also fixed the problem