The Floral Library is a flower garden patch planted by the National Park Service next to the Tidal Basin. Its first planting, in the spring, consists mainly of several varieties of tulips, but there are also daffodils and other flowers. Each garden bed has a sign indicating the specific variety of flower.
The tulips typically bloom a little later than the famous cherry blossoms, but it's not uncommon for them to be in bloom at the same time. So the Floral Library makes for a nice little diversion as you wander around the Tidal Basin.
As the bulbs fade, they're replaced by other flowers through the summer.
Photos of the NPS Floral Library
How to Get to the NPS Floral Library
It's located right next to the Tidal Basin, next to the northern end of the Maine Ave parking lot (the opposite end of the parking lot from the paddle boats). If you're trying to get your bearings, it's on the corner of the Tidal Basin closest to the Washington Monument. And it's across from the John Paul Jones Memorial.