I have 2 girls (ages 6 and 8) who I also struggle to keep stocked with age-appropriate, level-appropriate reading material. Neither of them like "heavy books" (Lemony Snicket gets no love here) and they both love "funny characters" so that skews our library a bit. I second a lot of the suggestions you've already been given (anything by Ron Roy and Roald Dahl especially!). A few others I would add (roughly in order of reading difficulty):
Ready Freddy series
Judy Moody series
My Weird School series
Bad Kitty chapter books
Clementine series
Pippi Longstocking series
Humphrey series
Frankly Frannie series
Mrs. Piggle Wiggle series
Emily Windsnap series
And when she's ready for longer/harder books (what I'd call Harry Potter level):
The Mysterious Benedict Society series
The Name of This Book is Secret series
Molly Moon series
Mousenet
Secret Garden, A Little Princess
Ella Enchanted series
Frog Princess series and Wide Awake Princess series both by E.D. Baker (these are a bit more "search for true love" than I like but the princesses are MUCH more proactive than the Disney Princesses so I let it go)
From the Mixed Up Files of Mrs. Basil E Frankweiler
Tuesdays at the Castle
Hoot and other books by Carl Hiaasen
39 clues series
For non- fiction, we've really enjoyed the "Who was ..." series. I think it talks about serious events in a very age- appropriate way and it does a great job of putting the famous people into the context of their historical times. We're also fans of:
"I Survived" series
Tales of Famous Americans and Tales of Famous Heroes
National Geographic Kids chapter books (ex: Tigers in Trouble!)
I say "we" because I try to read at least one book in each series before okaying them. But I can't keep up so I can only say that these books should all be age- appropriate for a 7-8 year old as far as I know. And I try to limit their exposure to poor behavior and "tween themes" so no Diary of a Wimpy Kid or Twilight books are to be found on their bookshelves.
I like to use arbookfind.com to look up reading levels because it also gives an "Interest level". LG is lower grades (K-2), MG is middle grades (3-5) and UG is Upper Grades (6+). That way I can save a little time by eliminating all the "UG" rated books right away.
Hope this helps a little! I know firsthand how much work it is!!
Kate, mama to
2DDs and a DS