Reddit reviews Music for Sight Singing (9th Edition)

A book like this will contain lots of rhythmic source material:
https://www.amazon.ca/Music-Sight-Singing-Nancy-Rogers/dp/0205938337

Lookout for these on kijiji at the end of the school year as you're guaranteed to find a 2nd year music major unloading one.

With exercises like the ones in that book, you can practice speaking rhythms (even using simple 'da da da' or 'ta ta ta' syllables) or clapping them out. And making sure you always do that with a metronome. I found this kind of instrument-free work really helps in internalizing rhythm.

For example, when you're learning a piece (say, as a guitarist), put your guitar down. Get a metronome going and try to clap the rhythm. Try to speak it out loud. Try clapping your hands or tapping your foot to the metronome while speaking the pattern against that steady beat. Move your body to the pattern in some kind of way. When you pick the guitar back up and look at that same bar of music, you will have another level of rhythmic understanding of it. It will be memorized, and internalized independent of any muscle memory.

Also, doing work like this daily will seriously enrich your rhythmic comprehension. Like comprehending written words–where you don't need to think about each individual letter and the entire word just is what it is–eventually, even the most complex written rhythms begin to indicate specific patterns.

Beyond that, you can start looking at songs in a dimension you never thought of before. Re-imaging a jazz standard as a waltz or in 5, looking into polyrhythmic work (where you keep two independent rhythms steadily moving), or even just changing the feel of a familiar song.

It's Ottman: http://www.amazon.com/Music-Sight-Singing-9th-Edition/dp/0205938337

Thank you for sticking with me (and hopefully some others) on this. One of my grandsons is 11 years old and I intend to help clear the way for him to become a fine musician. You have helped us both.