The following comes from a "member-supported public radio" site, and it shows how important the choice of textbook is:
If you were tasked with buying textbooks for a school, and your four best options essentially cost the same, how would you decide which one to buy? Wouldn't you ask, "Which of these textbooks will do the most to help kids learn?"Common sense should tell us that the choice of textbook is important, but clearly some people have other influences beyond common sense.
In reality, educators don't always get an answer to this question. Only a handful of states collect data on which textbooks schools choose, much less the effect of these choices on students' test scores...
Their takeaway: the textbook a school chooses can have a pretty big impact on students’ test scores, at least in math...
Also, education reformers, take note: (USC researcher) Polikoff said the measurable impacts of adopting this textbook on student test scores were as significant as — if not more significant than — the impacts of other commonly-touted-but-controversial policy changes, such as using teacher evaluations in layoff decisions or expanding school choice.
Two Algebra 2 textbooks ago, I taught Algebra 2. After the results of our standardized tests came in, my vice principal called me into his office. He wanted to show me my students' results. On a bar graph there was a very tall rectangle--it was the performance of my Algebra 2 students. Next to it was a significantly shorter rectangle, and it represented the performance of all Algebra 2 students at my school (including mine). Next to that was a very short rectangle indeed, and that represented the performance of all Algebra 2 students in our suburban district. Needless to say, I was pleased to have some objective evidence related to my teaching abilities.
Shortly after that our district purchased new Algebra 2 books. I was mortified when I saw the results of the next round of standardized tests. My rectangle was about the same height as my school's rectangle, and those weren't significantly higher than our district's rectangle.
Don't tell me that textbooks don't matter. If your reply is "you should get outside sources so that you can continue to teach at the high level you used to", then that's recognition that the current textbook isn't satisfactory. If your reply is that perhaps I got lucky that one year, that perhaps I wasn't as good a teacher as I had thought, then explain how my entire school's performance dropped so much.
Don't tell me that textbooks don't matter. I can do great things with good tools. Crappy tools don't work so well.