In my experience, having a teacher is the best way to go. You can watch videos on YouTube, but if you have any questions or need elaboration you can't ask the video because it won't respond. The ability to interact with a person is important because they can give a sense of musicality, how to express through your music, etc. in ways that a video can't. Teachers can also hold you accountable to consistent practicing, which is I find helpful.

For a while, I did what you're doing, and it seemed to work out pretty well for me... except there were always a few trouble spots in certain songs that I could not figure out on my own. As you move up in skill, the music will get increasingly complicated and it eventually gets to a point where you will need a teacher.

It is a good sign that you have the initiative to practice on your own and move forward with or without a teacher/resources, etc. I find it refreshing, and I do recommend a teacher if you find the music becomes more frustrating than it is enjoyable. I'm not too familiar with book series except Suzuki and Alfreds... both of which I dislike for personal reasons (though they can be useful resources). Just know that not all series are the same and they apply different learning theories that may or may not work best for you.

I'm curious as to what others have to offer on these various topics.

I agree with organtechnic that having a teacher is the way to go. I've tried teaching myself guitar several times, but have not yet gotten very far. I know I would go further if I had a teacher.

Ha! I know exactly what you mean. Practice time for me usually goes down the tubes during the semester. That means is that the time you do spend practicing is extremely valuable. Usually what my teacher and I work out is to meet once every two to four weeks when school's happening. That way I can sound like I've actually improved from the last lesson because I've had more time to practice than if we met every week (especially if there's a week or two where practicing is just not possible).

A teacher is only as good as the method and materials through which he/she chooses to guide you. If you spend money on the wrong teacher, you might end up more messed up than if you had saved the money and taught yourself.

I'm not sure what country you are in, but there is an excellent beginner series of books called Piano Adventures. This series gets students moving around the keyboard from the very beginning (no thumb stuck on middle C stuff). You can choose the adult books, or better yet, the ones for accelerated learning. The authors are Nancy and Randall Faber, who although maybe not so well-known, have put out what I have found to be a very user-friendly and pedagogically-excellent series. Google it and you should find out where to order, if this interests you. My students who only have enough money for a 30-minute lesson per week but want to advance by leaps and bounds on their own between lessons have had great success with this method being the conduit that quickly leads them to being able to play complicated classical repertoire far sooner than later.

If you have the time to practice 30 minutes a day, I think it would be okay to meet once a week. I meant that if practice time is seriously low to the point where you wouldn't be making sufficient progress after a week, then I'd meet every 2 weeks. Usually what happens in a lesson (for me) is that I play the piece, the teacher gives ideas on phrasing, articulation, advice on fingering (aka technical issues), attention to dynamic markings, weight distribution, wrist positioning, etc. Each of these topics can take several minutes to explain, demonstrate, and have the student repeat the form of musicality being discussed. We also talk about dynamics and expression not written into the piece. With all levels of music, it is not an uncommon practice to have the student learning more than one song at a time, so multiply that by however many songs you have and your 30 minutes will fill up really fast! Because my teacher is very excellent at what she does, I feel that the hour lessons we have are definitely worthwhile. Every time we meet I come out playing different than when I entered, or at least with different ideas on how to go deeper with the music for next time.

I'd recommend finding pieces outside of your music books that you'd like to learn. Generally speaking, a series will have a fair selection of songs, but if there's something you'd really love to learn and you feel you're able to learn it, bring it up with the teacher and they'll be able to tell if that's within your range or not. It also gives the teacher some ideas on what kind of music you like to play and helps him/her find other pieces for you when you need a break from the books.