Stanley - I have up to book 3 and it ends with irregular past tense. I believe the series goes up through book 6. If your students can carry on a short conversation then those books may be beneath them.
It generally takes me about a year and a half to get through one book but I use the book as secondary to everything else. I use the book for about 15 minutes in 2 classes a month. (For me the book is just a means to confirm the language in a written form.)
Let's Go jumps a bit from section to section. I don't think the book and workbook alone are enough. You'll probably need to make some eslHQ worksheets to go along with it
Spectrum readers are very good. I use the Little Critter series (grade 1) with some 5th grade students. They like it. They also have a writing series to go along with it and I'm going to have my students do one of those books next. Grade 1 might be too remedial but grade 2 should be easy enough but still challenging in spots.