Whether you buy the physical book or find a Shin Kanzen Master N3 Reading PDF download, the way you use it determines your success. Do not just read the passages like a novel.
Shin Kanzen Master: N3 Reading (新完全マスター文法・語彙・読解 N3 series — specifically the 読解 / Reading volume) is a widely used JLPT N3 preparation book series. It focuses on developing reading comprehension skills required for the JLPT N3: paragraph structure, main idea/detail inference, tone and register, connectors, and speed-reading practice with realistic exam-style passages.
Don't start at page 1. Go to the back of the book. Take the first mock test (untimed).
These physical importers sometimes sell bundles that include a digital code. If you buy the physical book, check the back cover for a download code for a PDF supplement (usually only the audio, though).