First off, you should find passages because they will allow for more words per minute. I would not use the guided reading books because right now they are not reliable. We used our guided reading books in 4th grade, but they all were quite lengthy so it seemed ok!
I like the books 3 Minute Reading Assessments by Timothy Rasinski. You can find it on Amazon. They have a 1-4 book and a 5-8 book. You can also download the DIBELS online, which also has reading passages, at least for 1st grade and up. Those passages have between 200-400 words each. My very best readers last year would read like 160 maybe... I can't remember. Anyway, he passages don't need to be really long.
If the children are reading short books, you will have to give them a long passage to check with, because you can't know based on those books, especially if they only have 29 words in the whole book. My guess is that children reading books like that really need to focus on decoding and making meaning, and fluency comes later, when they are reading longer books. I don't work on fluency so much with my readers who have those little books. We are just working on learning to read them!
Basically, time the child reading for 1 minute from a passage. If you have a BOOK you want to use, find one with about 100 words or more. You will have to type the text of the book out so you can follow along. You mark any word that they skip, replace, say incorrectly, you help them with, etc. If they repeat the word, or self-correct the word it's counts as a correctly read word.
Say the child reads 63 words in one minute, and reads 59 of them correctly. You only count the words read correctly. Divide the two numbers and you will have their fluency % rate. In this case the child has a 93% fluency rate, and that would be ok for instructional reading. I think
96% and above is good for independent reading and below 90% is too hard... but someone else may correct me on that!
You could take a trade book at the level your child is reading and see if it has more words than the guided reading books. If not, my guess is that the children are really too low to be doing this kind of oral reading work with.
Also, if you have a subscription to Reading A-Z they have fluency passages on it.
Here are some other websites with passages:
(Even if you don't have a subscription, this will explain to you about how to do a fluency passage w/ a child.)
You can sign up for this and print out their materials... it's A LOT of paper. We use them at our school. But, I think they also have practice passages you can print out... I haven't used them but I think
I saw them on the site.