The Redwall series by Brian Jacques is pretty long: ~40 books, iirc. They all focus on different characters, but there are lots of people that appear in lots, such as Martin the Warrior.
Do I have to read the Redwall books in any specific order?
I've already posted about this on another SE site. Let me try to go into even more detail here.
There are two obvious possibilities for the ordering: publication order and in-universe chronological order. These do not agree, but in most cases where they do agree, they should probably not be contradicted (i.e. if book A comes before book B in both of these orderings, then you probably do need to read A before B).
First of all, it's important to note that there are various subsets of "the Redwall books", which form loosely connected series within the wider universe. These are:
the Martin stories - set during the time of the legendary mouse Martin the Warrior, who makes ghostly appearances to help Redwallers in almost all the later books. These are:
I would recommend reading them in this order, but it's not really important. Mossflower was published first, and could definitely be read first, but then of course you'd have to view both the other two as primarily flashbacks.
the Mariel stories - set long after Martin the Warrior and long before Matthias, during the time of Joseph the Bellmaker. These are:
Read them in this order. They form a unified two-part story, the tale of Joseph the Bellmaker and his daughter Mariel.
the badger stories - I've lumped these together because they're all focused primarily on badgers and/or Salamandastron.
Read these in any order you want. They're not connected to each other, are set many years apart, and don't even have to be read together (I've simply grouped them loosely together because they share a common theme).
the main series - starting with Matthias and continuing on from there, these books comprise the majority of the entire list.
Read the first six of these in order. They're linked very clearly one to the next by common characters: Matthias is the hero of the first, he and his son the heroes of the second, his grandson features prominently in the third, Tansy is the heroine of the third and features prominently in the fourth, and the last three are linked by Cregga.
The last eight are linked loosely if at all (for example, the Taggerung is mentioned in Triss as a distant ancestor of Skipper). I would recommend reading these in order, but you won't lose out on much if you don't, or if you skip some of them entirely.
Having divided the Redwall books neatly into subseries and discussed ordering within each of them, the question remains of how to order the subseries relative to each other?
Publication order would require mixing them all up so that you never read an entire subseries in one go. You could do that (especially with the three badger stories), but I don't recommend it - it would decrease your appreciation for each subseries by itself.
Chronological order isn't the best idea either. You'll appreciate the Martin stories better once you've already been introduced to the world of Redwall and the mythos of Martin the Warrior. Frankly the best place to start is probably where Brian Jacques himself started: with the very first published story, Redwall. Although he hadn't quite worked out his world in detail by then, and some things you read there will jar with later books, it's an excellent introduction to the universe of Redwall, and definitely doesn't assume any knowledge from any of the other books (including those set chronologically before it). So here's the optimal overall order: