Since my Ph.D., I have occasionally received emails from publishers (that I've never heard of before) about writing a book. They all seem like mass emails that have my name and some title of a paper I wrote or the name of a course I've taught pasted in. The majority seem fishy and I just ignore them, but the most recent one may be a legitimate invitation. I'm not interested in writing the book they're asking for at this time, but I have to question whether I should continue to ignore all of these types of emails outright.
Are there smaller/up-and-coming academic publishers out there that are actively recruiting authors in order to expand their catalog? If so, should I engage with them at least for networking purposes?
I am specifically asking about books, and in particular textbooks, not journals. In searching through questions with publisher-related tags, I only saw similar questions about journals.