Hi there Angella, probably the first thing to try would be Decent Sampler plugin. It’s free to use and download after you have registered for a free account. I think there are about 170 instruments available on PB in the Decent Sampler format.
You’ll notice that I don’t give a direct link. This is because sometimes posts don’t get published until any links have been checked by Keyholders on Pianobook.
I don’t use Reaper myself, so can’t help with the precise details but here’s how it generally goes to get a Decent Sampler library into a DAW.
1/. Download the Decent Sampler zip file from the Decent Sampler site and extract the contents and install the plugin.
2/. Download a couple of Pianobook libraries in the Decent Sampler format and extract the contents to a folder of your choice.
3/. Open your DAW and scan for plugins and then load Decent Sampler.
4/. On the very first use Decent Sampler will ask for your account email and password and then ask where you have put your libraries. Navigate to the folder that you chose in step 2.
5/. When Decent Sampler loads click on FILE… and select Rescan Libraries.
6/. Click on FILE… and select Sample Libraries and then select a library or preset, which will then load into DecentSampler.
Hope that helps. If you do get more involved in it, you might look at other software and plugins.
By the way, Dave Hilowitz, the man behind Decent Sampler has a few instruments on his site as well which also come up on the plugin if you select BROWSE and SAMPLE STORE.
If you need more precise help post a new thread in the main forum as there is more traffic there and mention if you are on windows or mac.