for explanations and a basic introduction to 4u topics i recommend steve howard's textbook (found for free online, just search it up).
depends for modern. my teacher is the one that collates the relevant information + sources + historiography from various textbooks, sources and actual historical books and papers by actual historians into a booklet for each lesson/syllabus dotpoint, because no one modern history textbook is going to have all the stuff you need. but if your teacher doesn't do that, for modern i'd suggest just investing in the ken webb books if you're looking for the relevant info that you need + relevant historiography (to me, they're very good for revision and help me the most with structuring my notes).
if you're doing ussr for national studies, try to invest in michael lynch's books if you can. my teacher puts some pages from his books relevant to the dotpoints we're doing in class; good for in-depth knowledge and the books are structured like textbooks that are easy to understand. he's also a historian with good arguments so he's great for historiography. stuff from the hta journals and study guides are always helpful too.