twocats So I'm sure you can find old RCM books at a lower price (try eBay?). It doesn't matter since you're not taking any exams!
I did the grades consecutively but there's always some pieces that are borderline a neighboring grade
Yep, if you are not doing exams you can probably find the older editions of the book on Craigslist for a lot less money, although you may have to erase some markings. Because the syllabus for each grade is bigger than what is included in the books, a lot of pieces are carried over in the syllabus just not in the books.
To the second point, the book for each level typically provide a range of difficulty, with some pieces more achievable for early 'Grade X' and some for later when you'd be ready to transition up to the next level. Your teacher will be able to identify the most appropriate pieces or your current skills, although with adult learners they will no doubt (hopefully) take your musical preferences into consideration.