Not iOS specific. That's how AS works (though different clients have 
different levels of support for the GAL.

AS, itself, only supports a single address book. What Horde does is 
merge all of your selected address books (sync_books pref) into a 
single stream, so to speak.  So, as Jan said, your device can already 
have all your contacts.  If you are concerned about a way to filter 
out infrequently used contacts so your contact list is uncluttered, 
you can assign different categories to these contacts. Most clients 
provide a way to filer the contacts based on categories/tags.

The GAL is really designed to be, well, global. I really don't think 
we should change this to contain personal address book sources.
I know this was working at one point, at least on android.  Could very 
well be an iOS limitation. Please create a seperate ticket for this so 
I don't forget to look into it.

I don´t know if this is Iphone specific, but the Iphone keeps (with 
activesync) one Contacts Folder on the phone which is also present 
offline and it searches the GAL online. I have only a few Contacts 
which I need to phone often and lots of contacts I need from time to 
time but with which I do not want to clutter my Contactlist. I have 
those "rare" contacts sorted in multiple Adressbooks, so for me the 
ideal solution would be to have multiple adressbooks on the phone via 
activesync (which is as far as I have figured not possible). Second 
best would be combining the seldom used phonebooks into the GAL so 
that I can search this if I need one of those numbers. If this is 
technically complicating to implement or I am the only one who would 
benefit from it there is always the way via the web interface  so this 
is not really  a "must have" but just a "nice to have".

Another thing I have noticed (I dont want to open a new ticket because 
I dont know whether this is a Horde or an Iphone Issue). I f I have on 
the same Iphone 2 Exchange accounts which login to the same Horde 
server with different users, the second user will get the same sync 
data (calendar and contacts) as the first user not his own.

What does this have to do with GAL or even mobile phones? You can 
already sync several address books which show up as one on the phone, 
so you can search multiple address books just fine.

I think it would be a useful feature if it was possible to search in 
and use multiple adress books as a GAL. I leave my often used contacts 
in my main adressbook and would like to keep the  more seldom used 
adresses in other adressbooks so that they don´t clutter up the 
Contactsapp. I don´t know whether other users use similar setups and 
if this would be generally useful but I could imagine that it could 
also be used in a corporate environement (Multiple departments in 
multiple adress book, but searchable on the smartphone in one 
directory).