Rybkin, N. A. Textbook of Rectilinear Trigonometry and Collection of Problems (Uchebnik pryamolineynoy trigonometrii i sobranie zadach), 1927. In Russian.

Рыбкин, Н. А. Учебник прямолинейной тригонометрии и собрание задач. / 5-е издание стереотипное.
Москва — Ленинград : Государственное издательство (Госиздат), 1927.
175 с. : ил. Твердый издательский переплет, обычный формат (23х15 см).
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Rybkin, N. A. Textbook of Rectilinear Trigonometry and Collection of Problems (Uchebnik pryamolineynoy trigonometrii i sobranie zadach). / 5th stereotypical edition.
Moscow — Leningrad : State Publishing House (Gosizdat), 1927.
175 pp. : ill. Hardcover, standard format (23x15 cm). In Russian.

This 1927 textbook by Nikolai Rybkin is a classic of Russian mathematical pedagogy, representing the transition from pre-revolutionary educational standards to the early Soviet school system. Rybkin was one of the most famous authors of mathematics problem sets in Russia; his works were so effective that they remained in use for decades, undergoing dozens of "stereotypical" (unchanged) reprints.
The book provides a rigorous introduction to rectilinear trigonometry, focusing on the properties of trigonometric functions and the solving of triangles. True to Rybkin's pedagogical style, the theoretical material is concise and designed to be immediately applied to the extensive collection of problems included in the second half of the volume. These problems range from basic computational exercises to complex geometric proofs, famously known for their logical clarity and graduated difficulty.
Published during the NEP (New Economic Policy) period, the book features the characteristic typography of the 1920s. It was issued by Gosizdat, the central state publishing house of the USSR, at a time when the young Soviet state was standardizing its scientific curriculum to facilitate mass technical education. The text is supplemented by geometric diagrams and tables that were essential tools for students before the era of calculators.
For historians of education, collectors of vintage Soviet textbooks, and mathematicians, this 1927 edition is a significant artifact. It captures the enduring legacy of the "Rybkin method" and serves as a testament to the high standards of mathematical literacy that formed the foundation of the Soviet technical miracle in the 20th century.