Well, actually, I had recently bought a bookcase at a secondhand store that was going out of business and selling off its display shelves -- even though I had no books to put in it at the time. Because I knew I soon would. So Charlesia is sadly mistaken -- there is room for more than one new book in my house. That weekend I bought three.
Here's where/how I find lots and lots and lots of books to fill up my tiny home:
Shelf Awareness is a web site with reviews from independent booksellers around the country on newly published titles (and sometimes older ones as well) in all areas, both fiction and nonfiction. You can subscribe and get an email newsletter twice a week, though you can also just read the site.
As my friend Mary over at her blog, "Blahdeblahblah" likes to call them, I regularly visit my Plucky Local Independent bookstores, though they have sadly declined in number since I moved here lo these many decades ago. My all-time favorites are the University Bookstore just a hop, skip, and jump from where I work, and Third Place Books in Lake Forest Park. Both have new and used books, frequent sales, and friendly staff -- though the U Bookstore wins hands-down for "most effusive Truman appreciators".
A store I don't get to very often due to location (but which I hope to visit this coming weekend) is the Seattle Mystery Bookshop in Pioneer Square. Fabulous selection (again, both new and used).
Friends of the Library sales: these happen once or twice a year for the Seattle library, and take up a huge old hangar with well over 100,000 books at $1-$2 a pop (and half-price on Sunday!). The next one is in September and I have my strategy well mapped out after years of practice. I often come home (much to Charlesia's dismay) with 20-40 books from these sales.
Yard/garage sales: rarely do I venture in search of these, but I keep an eye on the listings for them in both the Seattle Times and on Craigslist in case there is one mentioning books that isn't too far off. My best score was when a local birder cleaned house -- that was a good yard sale indeed.
Speaking of Craigslist, they have a category for people selling books -- most of it is not anything of interest (though it can make for amusing reading, as in "1970 encyclopedia set only $150!"...dream on, folks...) but occasionally something interesting pops up there. Last year it was a family selling off their folks' estate, which included over 25,000 books and I can tell you, these were well-read, well-educated people. Good stuff. This year's score was an Encyclopedia Britannica set from the 1960s, and unlike the doofus above, it was reasonably priced -- as in, it was free.
One place I do NOT find books, nor buy books from, and never will, is amazon...for many reasons, though if you google "amazon" and "Hatchette", you'll get a good idea of their nefarious business practices. Steer clear, and keep supporting your Plucky Local Independents!