For beginners seeking an introductory textbook in number theory, "An Adventurer's Guide to Number Theory" by Friedberg is recommended for its accessible yet substantive content, ideal for non-math majors. For math majors at a sophomore or junior level, Underwood Dudley's "Elementary Number Theory" is highlighted as a solid starting point, particularly for self-study due to its approachable style and affordability. Other notable recommendations for more rigorous study include Apostol's "Introduction to Analytic Number Theory," Rosen's "Elementary Number Theory and Its Applications," and Stillwell's "Number Theory," which cover various aspects of the subject. The classic "Introduction to the Theory of Numbers" by Hardy and Wright is also mentioned for its analytic focus. Users emphasize the effectiveness of Dudley's book in clarifying complex topics and providing ample exercises, making it a valuable resource for foundational learning before progressing to more advanced texts.