You've got a bit a machine age modern look happening right now, a little bit deco/a bit 40's/50's ish. You need to pick images you like, for the reading room something scientific/botanical/architectural would seem appropriate to me, based your choice of a map on the other wall. Your guest bedroom might be nice to have something floral/still life/landscape etc. Needlepoint might be appropriate for a cosy space. The master is more modern, black and white photography, nudes, abstracts, things that are more personal might make sense.

http://www.art.com/gallery/id--a23630/john-james-audubon-posters.htm?ac=true

https://www.etsy.com/ca/listing/216776829/vintage-1940s-framed-landscape-oil?ga_order=most_relevant&ga_search_type=all&ga_view_type=gallery&ga_search_query=40s%20painting&ref=sr_gallery_29

http://www.atgetphotography.com/The-Photographers/Andre-Kertesz.html

https://www.etsy.com/ca/listing/462128066/completed-needlepoint-vintage-handmade?ref=market

https://www.etsy.com/ca/listing/459471694/vintage-daisy-needlepoint-completed-9-x?ref=market

https://www.etsy.com/ca/listing/156617710/dazzling-daisies-mid-century-barkcloth?ga_order=most_relevant&ga_search_type=vintage&ga_view_type=gallery&ga_search_query=floral&ref=sr_gallery_45

https://www.etsy.com/ca/listing/286227621/vintage-botanical-print-1803-loblolly?ga_order=most_relevant&ga_search_type=vintage&ga_view_type=gallery&ga_search_query=floral&ref=sc_gallery_3&plkey=fbc990e7c971747b875365ced3b39f80aa5160fe:286227621

http://www.delempicka.org/artwork/1918-1922.html

The secretary desk in the 2nd photo is too small for that wall. Maybe you can put it along the left wall (next to window).
A mirror could be used above the dresser where you have those photos

The painting above the bed (guest) should be centered on the bed..it looks off-centered in the photo

also, maybe you could reconsider the window treatments for the window that is facing the bed..I think the problem is the empty part above the window that makes it look like something is missing

Vacation Rental W5 · More Info

The master looks very unfinished without bedside lamps or sconces.

Check out minted.com for art.

Deserted Place Series 2 Wall Art Prints by Angela Simeone | Minted · More Info

Some white sheers in the room with the secretary desk will cover enough of the wall and then you can add some art

For the angled wall, here is an idea,,,it might be a bit much, but it might inspire you

Edina Tudor Estate · More Info

You don't always need art above the bed when there is a headboard. Notice our bedroom designs we included here? There is artwork on the walls, but not over the headboard. Also note the lamps on the nightstands and window treatments.

Azalea Cottage - Bedroom · More Info

Azalea Cottage - Bedroom · More Info

Driftwood and Seaglass · More Info