Readers do not want to be pitched book #2 or higher in a series.

You will see HUGE drops in click-through rate and sales if you actively spend time marketing book #2 or higher in a series. 

Imagine you are a reader, you are on Shepherd, and an author is promoting a fascinating book about spaceships and AI. But then you read the description and discover it is book #2 in a series; all that interest is wasted as no reader wants to start with book #2.

Might they then check out book #1?

Maybe, but you lost a lot of them. Any confusion and roadblocks you put in front of them will harm the interaction and lower the chance of a sale. Even if it is a stand-alone book, nobody will read that far to realize that. 

You are lucky if a reader gives you 5 seconds to grab their interest; do not waste it by pitching the wrong book to them. :)

Here is another scenario. 

Let's imagine that they read about your promoted book on your list at Shepherd. They are intrigued and click to Amazon to learn more.

They land on Amazon and instantly see the title of the book, which indicates it is part of a series and book #2. You lose their interest. You just spent all that time selling them on a story that is not the one they need to read first. 

If you are lucky, maybe they click to see book #1, but you will lose a massive portion of those people. Readers are not going to read for 5 minutes to find out is stand-alone either. 

I can't stress this enough :)

This is one of the worst marketing mistakes I see from authors again and again (the other one being an unprofessional book cover and an Amazon/Bookstore landing page that doesn't have the basics). 

You only want to promote book #1 in a series; that is their entry point. 

Once they read book #1, you market book #2 to them (and we will be helping with that long term, and Amazon already does a lot there to connect those). 

When can marketing book #2 or higher in a series work?

Only when you can segment the marketing/ads to focus purely on readers who bought book #1 in the series, but even then, you would only pitch book #2 to them (or the entire series, possibly). 

If you are doing a podcast and just released book #7, it is also okay to mention that, as that is a much more forgiving medium where someone listening is giving you their full attention. With a website, you have much less room to mess up as they are browsing and playing around. 

Kids' books matter less, of course, i.e. Hardy Boys #300 or a picture book series that is more a collection of characters (although Amazon labels the former differently).