‘oo’ has 2 sounds,
like in scoot /ū/ (lesson 14)
and book /oo/
/ū/ like in scoot, toot, balloon
/ū/ has rare spellings in common homophones: to, two, too
/oo/ like in look, book, hook
less commonly, /oo/ is spelled as ‘u’ (bush) or ‘o’ (woman)