2012.01.04 14:42 "Re: [Tiff] reading rgb values from 16 bits tif images", by Bob Friesenhahn
Besides that, does the LibTiff library supports now 32-bits images?
TIFF is a very flexible format and libtiff exposes most of its flexibility to you. By using the lower-level interfaces, you can use 32-bits integer, or 16/24/32/64 bits float images. As long as you are prepared to encode/decode the pixels, you could use completely arbitrary integer depths.
Unless your requirements are very specific or you have substantial time available to develop your software, you may be better off using a higher-level library which takes care of the lower-level TIFF details for you.
One possibility is my Magick++ C++ library, included in GraphicsMagick. Links to other useful libraries (e.g. VIPS, VIGRA, FreeImage, OpenImageIO) are included in the "Other Software Packages" section of the GraphicsMagick links page:
GraphicsMagick Maintainer, http://www.GraphicsMagick.org/