Tired, thanks for the info on Einstein!
This is definitely an old thread, but an interesting one.
My newly-9-yo has most recently read the Anne of Green Gables/Avonlea series. Looks like their Lexile numbers are 900s-1000s. He's also been reading and rereading The Mysterious Benedict Society and its sequels. He's definitely one who CAN read pretty much anything, but I limit what he gets in regard to content. He started reading the Narnia series at 7, but we don't do things like Harry Potter or a lot of the other fantasy books that are out there.
My 6.5yo is more of a closet reader. I know he recently borrowed "The Bobbsey Twins of Lakeport" from my mom's and read the whole thing. (Lexile lists other books in the series at 780-880.) That may be the longest full book he's read. He reads "Puppy Place" (~600s) and "Dinosaur Cove" (580-770) and "Magic Treehouse" (300s-700s) series with ease. He reads a lot of non-fiction, too, particularly about dinosaurs. He's a better speller than my 9yo, too, so I know he's reading a lot!
My 3.5yo is getting very interested in reading. My older boys read at 4 and I expect he'll do the same. He's asking what words are spelled with various letters on signs, complaining that he can't read words in (adult) books, etc., as well as knowing letters and sounds and such. Won't be long!
I like the Sonlight Quick Reading Assessment
and intend to go over it with them again, soon. I've thought it was pretty accurate in the past, though, as others have said, there are a lot of factors that go into reading level - and they change, frequently!