Do I understand correctly that you first worked through the Method book, and THEN the Pop Bass Lines book? Sounds like a good approach. I'm going to order the first EPBL book anyway, but I want to move forward.
I worked through Book 1 of the method book at the same time as "Easy Pop Bass Lines". I finished both books at about the same time.
Then I worked through Book 2 of the method book at the same time as "More Easy Pop Bass Lines". I finished Book 2 of the method book but was only halfway through the song book. So I am finishing up the song book before moving on to Book 3 of the method book.
Just like the method books, the song books are progressive. So the second song book is quite a bit more difficult than the first.
For example, I found the octaves in Crazy Train difficult for my fingers, and the tempo for me was a little quick. The rhythm for me was not intuitive on Heard it Through the Grapevine, so I got stuck for about 2 months! In many songs a person really has to think hard to find the best position for the fingers. Rhiannon changes strings and jump strings on the same fret with the pinky finger, and for me this is challenging.