chmlib is a small library designed for accessing MS ITSS files. The ITSS file format is used for Microsoft Html Help files (.chm), which have been the predominant medium for software documentation from Microsoft during the past several years, having superceded the previously used .hlp file format.
Note that this is NOT the same as the OLE structured storage file format used by MS Excel, Word, and so on. Instead, it is a different file format which fulfills a similar purpose. Both file formats may be accessed via instances of the IStorage COM interface, which is essentially an "acts like a filesystem" interface.