OK, folks, now you get to listen the the LIBRARY aspect of
MUSEUM activities [as opposed to the museum aspect of library
activities in a museum library].

Collection development is something we librarians have been dealing
with for a long time, in public, academic, and special libraries.
What materials do you buy, and why?  Problems get compounded in
situations like academia [or museumia?  if there isn't such a word,
there should be] when one has to reconcile a variety of personal
perspectives - the librarian
's, the dept. head's, the director/dean/insert-official-here -
with political realities [who's got the money & who's got the clout?].

Institutional mission is the best defense in deciding what to buy
for a library - whether it's a Danielle Steele novel or the letters
of Leonardo.  If you're in a museum of agricultural history, then
purchasing books on farm equipment is entirely w/in scope - because
it supports the educational/research work of other depts.  If you're
in an art museum, such purchases out of institutional budgets would
be questionable for personal research interests - but why are you
working in an art museum if your interest is ag history?

The real problem occurs in less-obvious [therefore more-insidious
forms].  Our museum, for example, is a museum of history and
natural history.  My predecessors as librarians were historians
and genealogists.  I just did a check on our science collection,
and the average pub date for our science books is 1975.  Should
our Natural History curator have to work with 20-year-old references
because his topic isn't "fun" for everybody else?  I think not.
You can't find a single reference to genetic drift in anything
we have in our book collection; ultimately, such information deficits
will affect the quality of our exhibits.

What to do on a limited budget?  One approach is to identify critical
segments of your library collection that are weakest, and pour
money into a different category each year.  This year, our library
target was collectible/artifact identification - we get a lot of
"What's this?" questions [logically], and we didn't have much to
support that activity.  Next year, it's vertebrate zoology - and
Louis Leakey will no longer be the darling of our physical anthropology
collection, b/c we'll have books later than 1972 on the topic.
The next year, a different critical area will be identified.  If it
turns out to be agricultural implements [not impossible], that will
be the area developed.

Focused collection development means that, at least at some time in
the present or future, someone's area of interest/expertise will be
addressed & updated.  After 4-5 years, it's time to re-evaluate the
categories to see what needs updating.

This approach eliminates the buy-everything-that's-advertised budgetary
approach, and also the buy-from-the-loudest-dept approach.  It also
looks at the institution's objectives & mission, not at anybody's
favorite specialties.

Don't know if this info is worth anything, but I hope it can help
solve some of these really sticky collection-development questions.

--
Fred R. Reenstjerna, Research Librarian - Douglas County Museum
Roseburg, Oregon  USA - [opinions expressed are my own, and I
stand behind every one of them]