In his estimable compendium, “For The Love of Hops: The Practical Guide to Aroma, Bitterness and the Culture of Hops” (Brewers Publications, $19.95), Stan Hieronymus lays out the history and science of the complex, aromatic flower of the twining vine known as Humulus lupulus.

As it turns out, the genus Humulus likely originated in Mongolia at least 6 million years ago, though the cultivation of hops for brewing can only be dated to between the sixth and ninth centuries, according to Hieronymus.

Fast forward to the 1970s, when California craft brewing pioneers such as Fritz Maytag of Anchor and Ken Grossman of Sierra Nevada made hops a major focus of American craft beer.

Today, heavily hopped, American-style IPA, once the fixation of hop heads and beer geeks, has gone mainstream as the top-selling craft beer style. Its bigger, bolder cousin, Double IPA, comes in second. And so called Session IPA, with lower alcohol content but plenty of hop power, is third.

Though there still many beer drinkers who are bothered by hop bitterness, flowery hop aromas and strong hop flavors are what make IPAs so appealing. The style is further enhanced by the fruity, citrusy, piney and resinous character of American hop varieties, including new and experimental varieties.

This week, Atlanta's Sweetwater Brewing Co. is set to debut 420 Fest Double IPA. It's a strong and hoppy (8% ABV / 85 IBUs) limited-release beer in the Dank Tank series that was brewed with two new experimental hops, Lemondrop and Eureka (just named this week), from the Hopsteiner Breeding Program in Yakima, Washington.

Brewed in honor of 11th annual 420 Fest , it will be served on draft at the festival site, and be available for a limited time in 22-ounce bombers and on draft around Atlanta.

Here are four more recent examples of the American IPA style to try now:

The Calling Imperial India Pale Ale, Boulevard Brewing Co. — Boulevard describes this new year-round offering as its "most heavily hopped beer ever, bursting forth with unmistakable tropical fruit and pine hop aromas and flavor supported with a slightly sweet malt character, tapering to a crisp, dry finish." It features a blend of eight hop varieties, including Mosaic, Equinox, Galaxy and Amarillo.

Tropicália American IPA, Creature Comforts Brewing Co. — Now available in cans in Atlanta, Creature Comforts describes its best-selling Athens-brewed IPA as "balanced, soft, and juicy," as "ripe passion fruit and citrus hop aroma lead to a full, fruit-forward hop flavor that washes over the palate, ending with subtle bitterness." Right now, it's the highest rated year-round beer from a Georgia brewery on the Beer Advocate website.

Hop Hash Double IPA, Sweetwater Brewing Co. — Brewed with bricks of pure hop lupulin, a highly concentrated form of hop resin, Sweetwater describes its creation as "the dankest double IPA out there. The pure hop lupulin, aka "hash" (the resiny stuff), meshes seamlessly with the two-row, pilsner and wheat malts, allowing it to deliver a pungent punch to the palate." On shelves since late 2014, it's already a big seller.

Hop Selection Double IPA, Terrapin Beer Co. — Terrapin describes its new seasonal series as "a celebration of our favorite hop from the current harvest." Ella, the hop used in the inaugural edition is an Australian variety with "an interesting aroma of spicy hoppiness backed with softer floral notes and a touch of anise, which is a contrast to the big citrus/tropical fruit and pine notes of many modern hops used in American craft beers."