Believe it or not Paul, the value has actually dropped slightly since you last asked this question back in 2013.
We're still waiting for the photos validating the "original" status of the 1888 through 1907 issues you said were to be published. Not until this is done can the true value of your collection even be pondered...
I'm very much looking forward to seeing the entire collection on display.
Identify, in accordance with my article, which format the volumes are bound in. Pictures will verify this to the forum.
If you do not have the original covers for the first 18 volumes bound within each book, then describe the measure of the pages (height x width), especially for the first 6 volumes (by volume), there are other markers for original versus reprint, but they would more than likely require a physical examination.
If the original covers are bound within the volume, show us pictures of the covers for several of the issues in the red brick series (first six volumes - and most valuable).
As far as potential value, you can use my article to determine the highest and second highest eBay sales for this set of "bound" volumes.
If your set is all bound exactly the same - the value is enhanced.
If your first 18 volumes are reprints, their value would be no more than $3,000.00 and probably less (many reasons for this).
If they are all originals with covers their value could approach $35 to $40 thousand (just the first 18 volumes). If they are originals (must be proved) without covers their value would be less than half of the "with covers" version.
This is "all speaking in generalities." On any given day, an auction might find the right set of circumstances, i.e. a bidding war, and sell for exorbitant prices. The opposite is just as likely for any given day. But a well researched, well advertised sale would most likely fall within the bounds I've listed above.
I hope you find these suggestions helpful.
Based on the photos you just posted with the announcement from the first issue, Volume I, No. 1, and Bearings Chart et. al. from Volume II, No. 2 your bound Volume, I and II combined, is most assuredly a (re) bound version of the 1964 reprints.
The paper gloss and the type margins are a dead give-away on this point.
Since you (apparently) don't have covers, that would clearly indicate whether or not these issues are reprints, the only other indicator would be the Volume I Title page.
A Volume I Title page would be proof positive of "originality!" The NGS did not, in any form (loose individual issue or bound volume) reprint the Volume I Title page. Though it's omission in this case does not in and of itself prove reprint vs. original, it is a strong indicator for reprint.
Now the key to assessing your entire collection would be to do a detailed analysis of each bound volume. Your father may have, at one point in his collection, decided to go with reprints instead of originals. In other words, some of the first 18 volumes may indeed be original. The NGS produced the reprints in two publications. Volumes I through VIII (1897) were first. It is possible your father had originals after 1897, or some other combination.
This assessment would truly have to be done in person unless you're willing to take a whole bunch of photos for each volume.
I would still like to observe the entire bound collection on display. It must be an amazing talking point with visitors to your home.