The Rosenbach Museum and Library in Philadelphia has filed a lawsuit against the estate of Maurice Sendak, claiming executors are withholding parts of the library that were earmarked for the museum in the author’s will.

During his lifetime, the renowned children’s book author and illustrator Maurice Sendak gifted the Rosenbach $1 million, plus he loaned the institution thousands of his drawings. Since 1970, selections of these works have appeared in more than 70 solo exhibitions at the Rosenbach. Museum officials had long assumed that the works on paper would remain in the museum’s permanent collection upon Sendak’s death in 2012.

Sendak’s will, however, stipulated that the illustrations would revert to his estate, eventually to be donated to his eponymous foundation in Ridgefield, Connecticut.

The Rosenbach did receive a bequest of $2 million cash, however, as well as an extensive collection of rare books—part of his 10,000-volume library, with a value estimated to be in the millions. And yet books are another sticking point.

According to papers filed in state probate court in Connecticut last week, as reported in the Philadelphia Inquirer, many hundreds of rare volumes promised to the Rosenbach library are being withheld by the estate, including valuable books by William Blake and Beatrix Potter. The lawsuit claims that the estate executors are playing fast and loose with the definitions of “rare book” and “children’s book,” and unfairly retaining certain titles based on arbitrary meanings of these categories.

Rosenbach officials have also expressed concern that some of these items will be among the offerings at an upcoming Christie’s auction of items from Sendak’s estate, currently being assembled for a sale scheduled for January 21, 2015.