Our primary books are the two Daily Ukulele books by Liz and Jim Beloff. They contain hundreds of familiar songs written in easy arrangements, so there is always something to play.
We also use the Ukulele Club of Santa Cruz Songbook or the River City Ukes Songbook. Ninety per cent of the songs mentioned in this blog will be from one of these sources, and we'll try to provide links or chords for anything that is not.
(The above are all unaffiliated links, provided simply to share what works for us.)