Writing two different books with some overlap I am in the process of writing a book on machine learning. I have also been contacted to co-author another book, on the topic of deep learning. The books are quite different, but they do have some overlap.
Is this going to be an issue?

I just e-mailed the editor to inform him, but I also wanted to have some independent advice. The book with the editor will probably go out first, the second one is a project with a friend we started a while ago and we are progressing slowly, and will look for a publisher later.

Is this a matter of possible concern? Of course I will not write the same things, but the overlapping topics will probably have a similar style if I am to write both books.
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.