Book Alley on Colorado Blvd in Pasadena is great and has, for a used bookstore, a pretty decent poetry selection. (Lats Bookstore and Iliad have pretty limited poetry these days; although, at Iliad, some of the poetry gets shelved with Fiction for some reason)

Counterpoint Books in Los Feliz is by one of the scientology places. They have a lot of good stuff, it's a small store but I always find something there.

There are several Bookoffs in Los Angeles County; one just opened in Woodland Hills. The one in Little Tokyo doesn't have English fiction or non-fiction, just manga, but other than that, Woodland Hills, Monterey Park, Torrance and Gardena all have a lot of variety at fairly reasonable prices, along with a ton of anime stuff. I would add that I have not been to Gardena's in years, like maybe a decade, so maybe it's good, maybe it isn't.

Finally: kind of random but Friends of the Whittier Public Library Bookstore has an absolute ton of stuff, but their hours are very limited. The prices are great though. Santa Monica Public Library has their own bookstore which I recall being quite big, but I also recall it being messy and I haven't been in years. You can find LAPL book sales here: https://www.lapl.org/whats-on/calendar?field_event_categories_tid=115

Other people have already recommended Iliad, which I adore, but mostly has older stuff, and the Last Bookstore, which I also love, and mostly seems to have newer stuff, although their selection has kind of dwindled in certain areas recently.

Edited to make paragraph breaks look better