This has been quite an interesting read, and seems to come up
every couple of years.  As my sister is the history librarian at
an Ivy League school, I asked her about this after the last round.
The basic answer is: unless the library really wants something or you
donate enough money to pay for the upkeep, the library can and will
do whatever it wants with your donation, which may include a quick
trip to the recycling center.

Her recommendation is to _offer_ your collection, and if you don't
get an enthusiastic response, take it somewhere else.  Make sure that
the prospective recipient knows that you want the donated materials to
be accessible and that if they're not ready and willing to make it so,
you won't bother them with the material.  The last thing a library or
museum wants is an un-cataloged bunch of old "junk" that they'll have
to store.

Which brings up the other point: if you have a themed collection of
anything, make a catalog, now!  That might be the difference between
a nephew saying, "All the old books go to the library book sale," or
saying, "Hello, music school, I have available a collection of 500
books on early 20th century instruments, may I send you a catalog?"

z!
Carl Zwanzig, who needs to catalog lots of things <sigh>