Absolutely the best and quickest is the Curt Sheller reading primer. It’s geared for high G but you can easily teach yourself the low G, A and B and the notes in between.
Only took me a couple weeks following his exercises and it teaches you to read the timing of the notes. It’s cheap. $15 or so. Then just pick up anything with standard notation like the Daily Ukulele Book and start playing the songs you know. You get faster and faster with practice. It really helped me.
To go further, to read chords, then take a basic music theory class like one on Udemy and you can start seeing how chords are built out of thirds. One you can recognize thirds you just look at the bottom note which you learned from Sheller and knowing the key, you know if it’s major, minor or diminished or you see a seventh added to the basic triad.
When you get to inversions it’s a little more complicated as are more complicated chords. Start with Sheller. It really helped me quickly.
P.S. If you or anyone else wants to become a better musician as well as a better ukulele player and you’re really serious, then it’s time to start studying piano. Piano for All by Robin Hall is a great course focusing on chord accompaniment and rhythm and learning to read treble and bass clef. Also, don’t be shy about taking a good ear training course or two. Enough said.