What have I got - rather- What do I have

I picked up today a small (3.75 x 5.5 in) volume which is somewhat
puzzling. It is a nice little half leather bound book, quite worn on the
binding but otherwise probably very good.
It contains material which I can find as having constituted 5 separate
books published in a series, early in the 20th century. This was Gowan's
Nature Books, published by Gowan and Gray of London and Glasgow.

The several parts (in order of appearance) are

# 1 Wild Birds at Home - 1905 (a stated reprint)
#5- Wild Birds at Home, Second Series -1906
-listed as "In Preparation on last page of #3
#2- Wild Flowers at Home, First Series-1906*
#3- Wild Flowers at Home, Second Series- 1905*
-A reprint ? Since last page lists through No. 4, or 3&4 may have
appeared simultaneously
# 4-Butterflies and Moths at Home- 1905
NO # Freshwater Fishes-1906 (catalogued by NLS as # 6

*odd though this reversal of number and date of publication may be, this is what
the title pages indicate.

Searching COPAC finds no such single binding, indeed relatively few of any
item. Holdings only at Oxford, British Library, Scottish National Library
and Imperial College. The last has a couple of them bound together and
one bound with some others in the series, but nothing like this finding.
This is probably less indicative of rarity than of limited technical
value.

My guess is that this is private binding of a personal set. But what
should I look for to determine this. I can find no binder's mark. There
is no bookmark or inscription.

Also, since this falls outside my active interest, I will probably at some
point sell it to finance my habit. Pricing seems problematic. Google and
AddAll finds single issues in this series at from 5 to 40 dollars, a set
of 21 at 475 euro. My suspicion is that for something this offbeat I
might do better on Ebay than shopping it around to my usual suspects.

Any comments?
Matthew Hill