Writing two different books with some overlap
First of all, as long as you cite your sources correctly, there is little concern regarding plagiarism. Secondly, It is actually quite common to see similar materials in different books written by the same author. This becomes more understandable if the topics are related (which in this case they are). The only misconduct I can think of is if you copy an entire chapter or section verbatim. Otherwise, as long as the core concepts and contributions of these two books are not the same, it is natural to have some overlapping materials (especially if its' purpose is to explain fundamental and introductory concepts)

In any case, informing the editor about this issue beforehand (as you have) is probably the most straightforward way to prevent any violations.