Redwall can be a hard series to read in order, as there are to ways you can arrange the books: the publication order and the chronological order.
I would recommend reading Redwall first and Mattimeo, and then starting at the beginning of the chronological order and reading down, though all the books work well as stand alone books, so reading them in any particular order is not nessicary.
Chronological Order:
Lord Brocktree
The Legend of Luke: In the Wake of the Red Ship
Martin the Warrior
Mossflower
The Legend of Luke
Outcast of Redwall
Mariel of Redwall
The Bellmaker
Salamandastron
Redwall
Mattimeo
Pearls of Lutra
The Long Patrol
Marlfox
Taggerung
Triss
Loamhedge
Rakkety Tam
High Rhulain
Eulalia!
Doomwyte
The Sable Quean
The Rogue Crew
