Problems in Linear Algebra (1978), a book of solved problems
I have the version  of this book in Spanish (Editorial MIR), the translated title is 2000 Problems in Linear Algebra. This publishing house (MIR) doesn't exist anymore but now goes under the name of URSS [0] and publishes modern versions of these same books, at least in Spanish and Russian. It has several books on problem solving, which was probably a pathway to demonstrating true "elite" status in eastern europe at the time (hence the known romanian, hungarian and russian way of teaching). It also contains several books on theory too, be it mathematics, physics or engineering. The important thing with textbooks on theory is, unlike the custom of modern textbooks to have few problems and often disconnected or that require to make a few "leaps" in the sense that the exposition does not clearly pave a way to solve harder problems, the amount of exercises is large and they frequently build on top of one another, thus making learning much more enjoyable in my opinion.Some books also worth looking at in mathematics:(1) Anti-Demidovich series(2) Combinatorial Analysis, Ribnikov(3) Problem series by Suprun(4) Probability Theory, Zolotarievskaya(5) Discrete Mathematics Problems, Evnin(6) Theory of Surfaces, Finikov... and several other problem book series for university mathematics.Engineering (mechanics):(1) Anti-Mescherski series(2) Theoretical Mechanics, V. M. Starzhinski(3) Strength of Materials, Stiopin(4) Problems in Strength of Materials, Volmir[0] https://urss.ru/cgi-bin/db.pl?lang=sp&blang=en&page=Bookstor... (apparently the headquarters are in Peru now)