Servers have filters for spam, it is in their heuristics to detect certain behaviors as spam. You get even sometimes very aggressive filters in the server of the people you are writing to. I lost a client (just doing a 1 on 1 mail, I've never done mass mailing in my personal activity/freelancing) because MS Hotmail/Outlook server (it seems their filters are more aggressive than others, for all I know, not the single data about it) simply I sent two mails with very short spam of time between them (seconds). Any user can be flagged for spam if doing this. But is a bit random. You might do it and most of the times wont be filtered out. It was us dealing with the gig specs, details, in normal exchange, but she thought I never answered her, and me that she has had a change of heart--- > gig lost. (and it was a portrait, my favorite type of project. Sigh) Sometimes is because the server you are using has already been blacklisted by some black sheep, even if most of its users are legit (but the entire server got black listed for complaints of several individuals about the bad user). There are many sites on inet for prestige about that, and sometimes is difficult to know if your server is fine or not (but can be checked). MANY times is something you are introducing in your mail code, or, the type of subject, that is triggering a HUGE red alert in the filtering systems, without you even knowing.
In the end, IMO, safest possibility is using some SaaS that is used to avoid these things and maintain a clean reputation of its servers. I know all these (without being an expert) things from my years at latest company, one of my tasks was handling part of that, and doing all the freaking 1996-like html that the mails can only have to be supported by a 99% of the mail reading systems. And that meant using : Campaignmonitor (one of the most friendly UIs), Mailchimp ( a very nice solution, big user base) , and Sendgrid (more possibilities if you are a programmer and system person, as allows certain integrations. I like it less, though. But is very well considered). I think most people go with Mailchimp, of the three...
Also, some mail apps, software, in the readers side, will add up to that server filtering, by as well "over detecting" as spam everything the thing believes to be spammy. So, is a lot of things that can trigger that which you have to be aware of. I always made the two versions : text and html+graphics versions, the 3 systems have that even offering you an auto parsed text version, but I preferred to always give it my last review and touches. Avoid all sort of javascript, btw. We never ever included attachments, as well. It can even be the graphics URLs, or how are you attaching the graphics. A lot of these things, if not all, are kind of ensured by those services to go in a safer way for you (and the readers). And... till certain number (not sure if I'm remembering right that it was around 500 addresses for several), several of these are free, with limitations. And then, there's different pricing according to number of readers, etc.
A mail newsletter is key for many type of products, today. It's current, is even more important than a bunch of social media. Now, for books, I have no idea (as most things writing related, am pretty new...)
There's a lot of literature on inet about what to take care of to avoid that filtering, but my advice is go with one of those companies. We'd rarely have any bounced / bad stats at all, mails would reach great all machines... Of course, my boss was an expert in marketing, and I was quite knowing my stuff (hehe) , she knew a lot of this things to avoid, so, she would always prefer to use one of those services, pay the money as it'd worth it.