A group including Crown Books Corp., its parent company, Dart Group Corp., and members of the Herbert H. Haft family, which controls Dart, is buying shares of May Department Stores Co., which owns The Hecht Co.

According to documents filed with the Securities and Exchange Commission, Crown Books has bought nearly 1 percent of the company's stock for $15 million.

May, however, said it had received notice under the Hart-Scott-Rodino Antitrust Act that Crown Books and certain affiliates -- Herbert Haft, Gloria G. Haft, Dart, Thrifty Crop., Trak Auto and Combined Properties Limited Partnership -- had indicated an intention to buy May shares.

The Hafts declined to comment on their purchase of 370,700 shares of the May company, with a company spokesman noting that Crown President Robert Haft and Chairman Herbert Haft "have nothing to say at this time."

The acquisition puzzled financial analysts. "I would think it's just an investment," noted Walter Loeb of Morgan Stanley & Co. Inc. Loeb and many other financial analysts have been recommending purchasing May stock, saying it is extremely undervalued given its assets, particularly its investments in shopping centers and a large apartment complex in Los Angeles.

With many analysts saying the company's stock was undervalued, there have been sporadic rumors over the past few months that May was a takeover target, although no company names have surfaced, analysts say. Yesterday, one Wall Street analyst noted that the Hafts could be interested in buying the company and then selling or refinancing its assets to finance the acquisition. Other analysts said they could understand May buying Crown Books, but not vice versa.

In a brief statement, May said it had "recently reviewed the interest of the parties and concluded that any significant purchases by them would not be in the best interests of the shareholders, employes or customers of The May Department Stores Co."

At a minimum, analysts said, Crown had made a good investment, purchasing its shares between Dec. 11, 1984, and Jan. 7, 1985. Since the first of the year, May's stock has risen 22 percent, closing yesterday at 45 3/8.

As of Oct. 31, 1984 -- its latest quarter -- Crown had assets of $104 million, with $3.4 million in cash and $50.8 million in short-term investments. For the first nine months of its fiscal year that ends Jan. 31, Crown had earnings of $2.4 million on sales of $72.8 million. For all of 1984, May Store's sales were $4.7 billion.