I know exactly what I like but it is so hard for me to find new books to read, and I finish about one out of every 20 weeks I do start. Those books I finish though I devour them.

I like non-fiction especially about things like genetics, medicine, etc. Not scholarly, though. Things like The Immortal Life of Henrietta Lacks.

I like anything about running, biking, swimming, triathlons.

I tend to like most biographies, I loved Andre Agassi's and Arnold Schwartznegger.

And yet it takes me months to find the next book sigh

I goto Amazon and look at the listings of books i've recently bought. The section there that shows what other customers who purchased the book purchased is great.

Browsing on Goodreads and Amazon are the main ways for me these days. Especially when I start following recommendations. "Best of" lists at those places are also things that give me good ideas of new authors to try (which reminds me I'm overdue to check out all of the "Best of 2019" lists that have been published in the last month). Goodreads does some other things that I find useful. Such as showing me when a new book came out by an author I previously read and liked. And letting me know what the "hot" (as in most widely read) books are every month in the genres I enjoy.

While there are many advantages to doing everything online now, I do miss the old days of hanging out in book stores and browsing. I could spend hours just going down the shelves and exploring. Especially big used/indie book stores with selections that went way beyond what the chains would offer.

I rarely shop for books online so when I feel like getting a new book, I visit my favourite bookstores and browse around. If I'm lucky, I might find a book or 2 that's of interest to me. If I don't, oh well, still have a ton of books at home that are just waiting to be read or reread.

Outside sticking to subjects or authors I really enjoy, many of my new book recommendations come from friends, colleagues or the students I teach.

Also, as mentioned by other posters, I'll browse local bookstores or see what Amazon recommends me based on what previous books I've bought.

In the niches I read, there are very few authors.

Nevertheless if I'm not familiar with a particular author, I'll do some googling to figure out whether they're worth reading at all. Frequently their formal books are compilations of their previous articles and/or lectures, rewritten to fit more coherently. (If they're really lazy, the formal books don't even rewrite/edit).

The decision to buy the actual paper book, has a lot to do with whether I want to print out several hundred (or thousand) pages and/or if it is a book I'll read more than once. If I'm likely to only read something once or twice, then it is easier to just read the original articles and/or examine the original lectures/seminars on youtube or a podcast.

Finding a new book is always a personal battle. There is nothing worse than taking a chance and then discovering its a dud. I lean pretty heavily on goodreads and amazon recommendations, and a general browsing of best of lists.

This year, I spent a ton of time around the holidays creating a reading list that should take me through a big chunk of this year, and that has been a big success so far. Doing all the work up front and just moving on to the next book is a great feeling.

I have a tendency to stick in and around the Horror/Mystery genre, so I have made some rules for this year. I usually have 2 books going at a time, so my rule is that my audio book must be some sort of educational/non-fiction/essay book, and the book Im reading rotates between classic novels that Ive missed out on and my normal reading(usually horror/mystery)

Ditto Goodreads and best of lists.

I miss going to the library and walking through the stacks. "Oh, that looks interesting!"

I look under the potato and there they are!

Recommendations and random comments.
Like, just recently someone online made a random comment about an official sequel to the original War of the Worlds novel, authorized by the Wells estate. So, after I'm done with my current library batch, I'll get that.

Our library has been open for a while, we usually use online reservations to have them set aside so we don't have to spend time in the stacks, but they've always had a section at the front for new releases and I'll give that a quick look whenever I go in for a pickup.

I read mostly fiction, so this applies to that.

Nowadays:
Book of the Month Club
Bookbub (e-books)

Previously:
Browsing bookstores/libraries
"If you liked this" feedback
My library system's annual book sale
Random stuff at the library (online reservation/quick pickup)

I get a daily email from Book Riot that lists ebooks on sale. Lots of duds, but I’ve gotten plenty of good stuff, too. But obviously this depends on whether you are fine with ebooks or require a physical copy.