Visual: Just look at that pair of expectant eyes in the middle of the word, oo. At the same time, you may see the word as like a soap bubble: you blow it from the bubble wand b; it floats along o o, but ultimately pops, perhaps when it hits something, n.
In the mouth: Saying this word is also a bit like blowing a bubble – or a kiss. You make the puff of air from the lips and hold them puckered to blow, finally diverting the air through the nose by stopping the flow with the tongue.
Echoes: You might think of Daniel Boone or Pat Boone (or you might not); you will probably get the tastes of boo, boom, and bone. You might think of a boon as a sort of benny, and that could bring you to notice the modest resemblance between boon and benny.
Etymology: Boon is not related to benny, but it is to bene – not the Latin word for ‘good’ but the now-obsolete English word for ‘prayer, petition’. The noun boon comes from the Old Norse sister to that word; a boon was originally a prayer request, and then a thing granted in prayer, and then any good thing granted – or simply a big boost, maybe a mini-boom. This is also influenced by the adjective boon, which means ‘good’ or ‘convivial’ (as in boon companion) and comes from French bon.
Collocations: The verb to go with boon is generally grant (although one may first ask for one). Boons are often great or tremendous; they may be economic or financial. There are a few little-used compounds, generally relating to old English agrarian customs: boon-day, boon-man, boon-work, boon-ploughing, boon-loaf, boon-shearing.
Overtones: Although this word starts with boo, it really draws on boom and boost and has the warmth of a bosom. It is a consciously literary word, but not an ornate one; it suggests formality, but the formality of either a robed supplicant or a spokesperson in a business suit.
Semantics: Probably the best synonym for boon, as Visual Thesaurus points out, is blessing. That relates back to its original use and yet also draws on the looser sense for many current usages: “Increased tourism has been a real [boon/blessing] for this small town.”
Where to find it: You probably won’t find it in casual conversation, friendly emails, text messages, et cetera. But you can find it in newspapers, articles, and assorted other non-fiction; you may even find it in fiction – novels and plays – although my suspicion is that it will be more common in the older ones.