Bilbo’s riddles with Gollum sharpened his wit for riddling with Smaug.  The Chapter 8 references to riddling are about recounting the story of “Riddles in the Dark”.  Tom Shippey classifies riddles as one type of truth found in Old English poetry, “the basic rule of which is that all statements in them must be true, but also misleading”.  I suggest you begin with his essay when you launch your own study of riddles and the many papers which have been published on the influence of Old English poetry on Tolkien’s work.

• 05.024 because he had not had time to think of a riddle.
• 05.052 but Bilbo asked another riddle as quick as ever he could,
• 05.054 It was not really the right time for this riddle,
• 05.062 but Gollum thought it was a riddle,
• 05.076 and after all that last question had not been a genuine riddle
• 05.098 Not a riddle, precious, no.’
• 05.022 It likes riddles,
• 05.022 Riddles were all he could think of.
• 05.029 and nearly bursting his brain to think of riddles
• 05.037 everyday sort of riddles
• 08.092 asking riddles
• 08.125 riddles and all,
• 12.056 beginning to be pleased with his riddling.
• 12.058 No dragon can resist the fascination of riddling talk
• 12.087 from his riddles added to the camps

Shippey, T. A. “Approaches to Truth in Old English Poetry”. University of Leeds Review 25. 1982.  PDF of reprint.