Yes, both books can be used randomly, #2 isn't harder than #1, just different. #2 has a bit more Bill Monroe (1960s-1970s) in it, and thus carries the subtitle '16 songs from yesteryear', while #1 more strictly is '16 tunes from the golden jazz age'. But once you play them, it's hard to say which tune belongs in which book. #1 has info on Martin and National ukuleles, #2 on Knutsen, Hollywood and Ludwig (banjo) ukuleles.
Oh, and #1 is still for sale, although you have to look for remaining stocks, like here:
http://www.mercatinodellukulele.it/...itstart=0&option=com_virtuemart&view=category