I was on one of my searches when I came across the other forum that you've posted this on, and in some ways was amazed to see a post suggesting that ...
Well, it's not handwritten, simply Fraktur. This 2nd volume is about insects only (almost 700 pages, BOAH)I imagine that they have only seen the images that you've posted here, so I'm at a loss as to how they came to that conclusion. The only thing that I can think of is that they didn't follow this link, instead just thought that it was an early Brehms Tierleben.
I am sure that you can tell the difference between watercolour and print, and handwriting and print.
You have, in my opinion, a unique pair of volumes there and I believe the value lay in who the author and illustrator was. If they are not particularly collectable in their own right then they may not be worth a fortune, the illustrations that we can see are competent but not necessarily works of art. I do hope they turn out to be true attic finds!
KCs points are also good ones.
As I mentioned earlier, I have a passion for old books, and I suppose that I'm fortunate that they can be bought fairly inexpensively. My main interest is in old 19th c Dictionaries, I just love to see how language has developed as well a to seek out those words that seem to have just disappeared from common use (as well as from modern dictionaries).
A while back I was given a gorgeous two volume, massive, set of Funk & Wagnells 1907 Standard Dictionary of the English Language. They were an 'unwelcome' gift and so I didn't really attach any fondness to them. I do know that they cost c. $90 over 10 years ago, and last year my local antiquarian bookshop (where I get many of my finds from) offered me the equivalent of $30 for them, despite being in immaculate condition (despite being unwelcome presents I couldn't bear to part from them for such a small amount). These are obviously not in the same category as the Brehms volumes, having been mass produced, but it still saddens me that such beautiful things are so undervalued nowadays
