<p>That’s right; those topics lack depth because:</p>
<ol>
<li>the average high school student struggles with those at a fundamental level</li>
<li>precal has to cover too much material to ever go in depth.</li>
</ol>
<p>For Number Theory, I would suggest Introduction to Theory of Numbers by Niven. It’s pretty solid but it doesn’t cover everything - no NT book can. In my view, I think it’s better to study Linear Algebra and just do NT problems instead of just straight theory because the concepts are much clearer through practice. The AoPS NT book is also great I’ve heard - actually they’re all great.</p>
<p>I don’t know what to suggest for conics, as that is a pretty specific subject. That topic is covered in differential geometry in college, and I’ve never read anything about it [only a HS senior]. For Seq.'s and Series, I’d stick with learning calculus. BC calculus introduces series, and later maths expand upon the ideas of power series [taylor, maclaurin mainly], and eventually develop to fourier and laplace series/transforms. </p>
<p>Don’t take this the wrong way, but I feel as if you just know several terms without any true sense of what you want to learn or how/why you want to learn it. For example, you mentioned that your pre-calc book lacked material on mathematical induction. For the most part, mathematical induction is a concept. I could teach it in three sentences and an example. The main way to develop that is by doing problems, and no book is devoted entirely to induction [or maybe there is, haha]. </p>
<p>I would suggest you stick with Spivak and work through the book. You said you glanced through it and found non-calc stuff at a rigorous level. This is a bad way to read it. I would suggest you literally read every single word in the book and understand where he’s going with it. Chapters 1-2 [and maybe 3, I forgot] work with non-calc stuff, but he goes into calc concepts pretty fast. </p>
<p>Nonetheless, being able to solve the problems in Spivak will give you the background you need to handle any math course, period. It’s good that you, as a high school student, have picked up a book most college students wouldn’t touch, but this isn’t Edwards & Penney or whatever they use to teach calc nowadays. That book is a read, recite, rewrite, and rediscover kind of book. You teach yourself with his guidance, but you have to do it diligently. Instead of reading on many topics, just work through this book, and you’ll be fine for everything else.</p>