Here is the stereotypical undergraduate engineering math curriculum in the US. Of course every person/school is different, but this is a useful reference point for initial awareness. (The last sentence is actually covered by the single word "stereotypical" but some of the caveat Na...police need to be told twice.)
SEM 1: limits, differential calculus, start of integral calc
SEM 2: more integral calc (trickier methods), chapter on series, chapter on diffyQs. YOU ARE HERE.*
SEM 3: multivariable calculus
SEM 4: Ordinary differential equations (some repetition of chapter from SEM 2, but more detail). Typically most (even basic/applied texts) have a bit too much to cover in a semester and later topics (especially systems of equations, Laplace transform, and series methods) will get an abbreviated (even omitted) coverage.
SEM 5: "Engineering Mathematics" or "Methods for Engineers". This is the most vague, but a typical course would be about a half semester of linear algebra (matrices) and a half semester of PDEs/Fourier series. Both of these are topics that could easily get a full semester each, even for an applied engineer look...but few schools give that much time. [You do have a huge slew of foundational mechE courses like statics, dynamics, fluids, thermo before you even get into your aero majors courses where you learn lift and drag--there is just limited time in the schedule.] Although some will do a separate formal linear algebra requirement of a semester, in which case, the engine math course may go deeper into PDEs. I have also sometimes seen physicists (but not engineers) doing a two semester "methods for physicists" which is basically like two semesters of engine math. Typically undergrad engineers (especially non EEs) will not have complex analysis, even "for engineers" as part of their required sequence.
It is possible to get specialized texts for all of the semester 3 to 5 topics. However, the Kreyszig text covers them very well. It was written by a mathematician and actually teaches math, tricks and concepts/derivations (not just formulas in passing while learning some physical topic), but by someone from the prehistoric era when mathematicians liked engineers and physicists. It's a very good middle ground between the sort of unsatisfying explication you get within a technical topic (by people not that great at math OR teaching math) and the proof-centric (but not caring about tricks/techniques) approach of the pure math types. Within it, you will have a clear "about semester" coverage of the following:
- Multivariable ("vector") calculus
- ODEs (K reverses the order and has it first...nothing wrong with that)
- Fourier series and PDEs
- Linear algebra
- Complex analysis [This is "extra" above the minimum normally required for aero.]
If you pound all that out (and I mean do it like a martial artist, learning skill after skill, doing each drill), you'll have a very, very strong basic coverage. Better than the normal engineer. Of course you can go to even greater applied depth with specific texts and/or do theory side trips. But I would start with this baseline FIRST. Can build on the foundation later, if needed/time. [You do need to learn slip and drag and Reynolds number and such sometime!] I would just use Kreysig since it is a single book, to lay this baseline. At least it is self consistent. While still also being very easy to pick certain sections to work independently. [Other competitor books probably fine too. But, stay the hell away from Arfkin and Weber...it's a mess--see the Amazon reviews (and I have it)...physicists trying to write math methods and reads like a laundry list, not an integrated teaching book, really.]
Some other math topics, not on the main calculus path, but very useful for engineers. I have listed them in priority order. (Also I would prioritize (1) and (2) below BEFORE complex analysis, for an aero engineer.):
Probability and statistics ("probes and shafts"). Do a basic survey course. K has a chapter on this, if you want the fast version. But maybe worth it to do a simple, applied text/semester on its own. This has some of the "ODE problem" in that the "texts that sell" have more like 1.5-2 semesters of work (yes, even in a simple applied introduction) and teachers will skip many sections to breeze through the topic. And it is possible to go into much more depth on TOP of what a basic intro would do--people do Ph.Ds in it! (C'est la vie.)
Design of Experiments, Quality Methods, etc. May have a non-intuitive name. But the key concept is (grossly simplifying) how to design and evaluate partial factorial examinations of complex, poorly understood phenomena (multivariable and nonlinear, on steroids...i.e. real life, engineering, business, farming, social topics like crime and engineering, etc.). The classic text here is Box, Hunter, Hunter but there are other good ones as well. May be better to take an actual class here, versus self study, as you get access to a lab and perhaps university copy of helpful statistical computer program. If you are going to go get a job in industry, this might even be more useful than the basic probes and shafts...and there is a little overlap.
Numerical methods. This is probably overkill for the average engineer to do a specialization in. But if you are going into grad school would be nice to actually spend a concentrated time on the different methods and understanding their error characteristics and computation efficiency. K has a chapter or two here, but I would probably look at a course or self standing text. But again, if you never get to it, you can still probably hack away as needed ad hoc.
Perturbation methods. Bender and Orzag is the classic text. Really not needed unless you are a very high end (mathematically inclined) aero Ph.D. But it is sort of "more calculus".
Few other text recommendations.
A. I am a huge fan of Schaum's Outlines. Very clearly written (not too much text...get to the point, simple wording). Lots of drill problems AND solutions. And implicit "teach by do" approach. They are economical, not like the normal textbook market. Order one whenever you have an issue with a regular text, hard to understand teacher, or just want more drill problems. Can do it ad hoc as needed by topic.
B. I am a big fan of the Michael Lindberg engineer license prep text books. The EIT reference manual essentially covers an entire undergrad engineer curriculum (all the statics/fluids mechE crap, but even basics in circuits, chemistry, physics, math, econ etc.) WITH DRILL PROBLEMS [spend the extra few bucks and buy the complete solutions manual also]. All in one book (with thin paper admittedly). I have "literally" run circles around engineers (despite lacking a BSE) because of all the quick reference info in this book. (OK, figuratively running the circles, but I was extremely tempted to warhoop around their desks.) I don't know about aero, but the Lindberg mechE prep book (I have it) is of similar quality to the EIT volume.
*From your question (i.e. not knowing the landscape), comment answers, and questions on the main math site, it looks like you are about at the end of semester 2.