Many years ago when my grandparents died I found thousands of letters, and other items in lesser degrees like medals, patches, and several Nazi items like pins, armbands, some type of skull flag, and other stuff from when my grandfather was in WWII. My grandmother had five brothers (she was one of 13 and based on the letters there were 5 brothers in various theaters) so I'm not sure what stuff came from who. I do know my grandfather was in the 451st Squadron based on a 1977 directory I have. I believe this is the 451st Bombardment Group based on patches I have and the directory. While most of this is my family history I feel that some items are better donated to a historical society. Where do I start to look for some place that would take some of these items or are they too common to be donated?
I recommend that you scan everything. Transcribe if possible. Keep copies of the scans and transcriptions, and then find a repository that might be interested in this material (together with the scans and transcripts). You don't mention the continent/country: that will help direct the best place for this material. A story as large as WWII will need as many voices as possible to represent what happened. Your material has value.
All letters and diaries are unique and never too common to be donated! Generally I'd recommend donating these sorts of materials to a local historical society/archives in the area where your grandparents lived. If either of your grandparents had ties to a particular university, the university archives may also be interested. The Society of American Archivists has a little pamphlet-style webpage for family donations that talks about shopping around for a home for your things you might find helpful.
If you would like to PM me with more specifics I would be happy to try to make you a little list of organizations to contact.