I don't know, withoutcolor. That all depends on what you consider motivational and is also Buddhist in nature. In Buddhism the motivation usually comes from within the person.
Although I don't particularly care for his teaching style (a bit too syrupy for me), you might want to look into Thich Nhat Hanh's .
Or Shunryu Suzuki's .
These aren't oriented to Theravadin practice necessarily, but I'm not sure, other than a book like the Dhammapada, what to suggest. Most good Theravadin books tend to be dry and practice oriented, which is what I meant by "the motivation usually comes from within the person" quote above.
Otherwise, an author like the psychologist Paul Fleischman in his book might suffice.
"The gift of truth exceeds all other gifts" — Dhammapada, v. 354 Craving XXIV