How do I choose?
Frankly, that is up to the authors. Yep. Whether or not I purchase a book is entirely dependent on how invested the author was in their story. By that I don't mean how much money they spent on book covers or publicity or conferences. I'm talking about how invested they are in their characters. Do they care about them at all?
There is a certain series of books I've loved in the past. I really enjoyed them and re-read them frequently. I was quite excited when the latest was released a few weeks ago. So far, I haven't made it past page three. The first few pages are obviously a set-up for the rest of the story, but I don't care what happens to the characters. I don't care!
I'm pretty sure the author doesn't either. Now, I know he's a more than competent writer. But this book? It's...cardboard. He's not invested in these characters. I get it that they're the bad guys, but that doesn't matter because a truly bad guy requires investment in the character. We have to care enough to want him vanquished. Think about Darth Vader from the Star Wars movies. He was the guy we loved to hate. We were invested in his demise.
A lot of authors wonder why their books aren't selling. They moan and groan over their non-existent royalties, even though they have fifty or sixty books published. Perhaps, just perhaps it's because they're not invested in their characters.
I buy mostly series. I freely admit I like following the characters, watching them grow and change as they face different challenges. I specifically like series that center around a group of characters that interact. If a romance is involved, that's okay, but the interaction of the group, the relationships revealed is what draws me to the series. Consider the television series most people invest their time and interest in. Is it the romance? Not likely. Mostly, it's the continuing cast of characters that keep people involved.
The writers and producers for a series are involved and invested in their cast. When they introduce abrupt departures and introductions of new characters, we get all riled up and annoyed. It's the same for a book, series or standalone. We invest our time and interest in the characters. When the author doesn't, it seriously annoys the readers.
What happens then?
We spend our book dollars somewhere else. I invest in several series that might not be prize-winning writing, but I am absolutely certain the author is well and truly invested in their stories. They care about their characters. Because they do, I do.