The Building Splunk Solutions books provide examples of building custom user interfaces, but they're a bit dated. PDF copies of the first and second editions used to be published on dev.splunk.com and may still be available.
If you have a Splunk support agreement, your sales engineer or account manager may be able to send you a hard copy of the second edition. Alternatively, you can buy a copy of the second addition from Amazon. Just note that it was published six years ago.
The Splunk Enterprise SDK for JavaScript is up to date, but the examples aren't as rich.
