Standalone mysteries: I love Agatha Christie. I know she’s old, but some of hers are amazing. In no particular order: The Mysterious Affair at Styles, The Murder of Roger Ackroyd, Murder on the Orient Express, Curtain, Murder on the Links, her play The Mousetrap, etc.
Historical mysteries: I like Anne Perry . Also, forgot the author, but The Hangman’s Daughter was good.
Supernatural mysteries with horror elements: Not a lot of horror, but a great series (not too long) of cozy supernatural suspense with mystery subplots are the Chintz and China books by Yasmine Galenorn. She (and previously her publisher did as well) calls them cozy mysteries, but really the mystery is the subplot not the main plot. They’re still really good, though.
Resource books: The Howdunit series by Writer’s Digest. I have their Poisons book, the Forensics one, and the Police Procedure. Most of the “how to write mystery” books are . . . well, maybe 5% specific to mystery writing and the other 95% about writing in general, so I haven’t found them very useful.
My general advice: Unless you are doing something really genre-bending, you probably want more than 3 suspects: 1 for the police (that the reader and MC know didn’t do it), 1 for the MC (that the reader knows didn’t do it), and at least 2 others for the reader to have to choose between.
Personal strategy: I generally do NOT know who did it when I write mysteries. I develop and treat all real suspects (not #1 or 2 above) as if they were the real killer, and am surprised myself when I find out/decide who had to be the real killer. Not everyone needs to do this, but I find it an effective way to avoid authorial bias (i.e. my knowing who did it subconsciously affecting how I present them on the page).
Hope this helps!