Ken,
Examine what you have and categorize the books. Consider donating those that deal with specific architect's works to Association's that are attempting to preserve not just historical information, but doing so in a manner that is encouraging any and all to research as well.
For example, at last week's PBA Gallery auction, a copy of Tilly's booklet "Planning a Golf Course" was sold for $14,400. Obviously the availability for a reasercher to examine something of this value and rarity (It was one of only four known copies) is limited in the extreme. This week, though, the Tillinghast Association website is going to place the full-cover images of a scanned copy of one of the four on our website, thus making it available for all to examine.
If you feel that breaking your collection up in this manner is too time-consuming or just doesn't work for you, what about donating it to a University archive where they have a strong Landscape Architecture program? Or one that is a member of the PGA of America's golf course management certification program?
You might also consider quietly approaching some on here who you know do serious research and ask them if they might be able to put some of them to personal use in their own researches.
I would not suggest donating them to either the USGA or PGA libraries as they most likely already have copies of the ones you have.
You could also approach your own state's golf association and see if there is interest. Be careful doing this as many, if not most, have neither the space or interest in maintaining any serious archive for research.
Finally, you could simply list them on this site and ask for PRIVATE emails from those who have an interest in them. Take the money that they may be willing to pay and either donate it to the site or to another deserving charity.