Personally, I think the intro to number theory is a great book, and reading the first few chapters of introduction to combinatorics, as well as getting an Olympiad primer for past papers, is a good idea, well, At least it's what I'm doing. I also have the Euclidean geometry one, but only a few chapters are accessible to me, it's a tough book, the imo team did every question from it to improve geometry one year. To be honest, at bmo1 u need only circle theorems, congruent triangle, basic knowledge, and basic trig. So number theory, primer, combinatorics I'd recommend. Oh and crossing the bridge I haven't got. I don't think it's based on competitive maths. That being said, I'm sure it's a good read