u/ArrowDiver

Now we have random additions years later that have things like Percy pissing his pants! Thanks, Rick!

Btw, the above is still on his website: https://rickriordan.com/about/frequently-asked-questions/

And yes, the recommendation letters are not a "separate series". They are considered direct sequels to the first 5 books and a part of the same series. They are also marketed and listed as such on his own website: https://rickriordan.com/series/percy-jackson-and-the-olympians/