Spring might be the season to spruce up your library for a rainy day. Four book sales coming up in the next two months could do the trick. The AMERICAN WOMEN'S OFFSHORE SAILING ASSOCIATION will hold a book, plant and bake sale this Saturday and Sunday in Ellicott City, Maryland, in the parking lot behind the post office at 8267 Main Street. Hours both days are noon to five. Take U.S. 29 north to Ellicott City and look for a bright spinnaker floating in the sky. The SMITH BOOK SALE, featuring 50,000 books, runs from Friday April 16 to Monday April 19 at the Towson Armory, corner of Chesapeake Avenue and Washington Street in Towson, Maryland. The last day, Monday the 19th, you can take away all the books you're able to cradle in your arms for $2. To get to the Armory, take I-695 to the York Road Exit, head south on York, go right on Chespeake Avenue, and make a left on Washington Street. Call Vivian Shephard at 301/435-2554 for further information. The week-long VASSAR BOOK SALE, which, at 100,000 volumes, is easily the biggest of the season, kicks off on Thursday April 22, in the Pension Building, at 440 G Street NW, and runs until Wednesday April 28, when buyers will be invited to stuff shopping bags with books at $3 a throw. Among the events of this yearly extravaganza, which attracts book dealers nationwide as well as casual collectors, are family days Saturday April 24 and Sunday April 25, featuring free balloons for kids and a flea market for their parents. Hours are 10 to 6 weekdays and 10 to 5 weekends, except the first day of the sale, Thursday the 22nd, when the commerce continues till nine at night. And look for the BRANDEIS BOOK SALE sometime late in May. At press time, the organizers were still cllecting books and searching for a location. Call 229-9143 or 762-1211 for information or if you want to donate books.