How should I react to emails from unknown (to me) book publishers? 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.
Get used to this: you will receive one or more spam emails for every single piece of work that you will publish, whether thesis, journal paper or conference paper.

The senders are predatory publishers: press the del key as fast as you can!