As the title say letter = number . You have to find out each word to solve the puzzle. Good luck .
5 8 20 1 9 12 15 18 19 20 25 3 3 9 12 19 20 25
1 1 3 5 19 6 15 4 5 9 9 14 20 20 25
8 9 19 1 12 19 20 2 15 23
5 8 20 3 4 5 5 11 12 16 19 1 2 4 14
2 3 5 8 13 15 15 19 19 4 12 15 1 3 5 12 16
5 8 20 1 5 7 13 18 19 21 22 1 9 12 18 20 21
5 8 20 5 19 19 19 21 24 1 5 9 13 16 18 21
5 8 20 5 5 7 11 18 5 5 5 9 14 16 18 18 18 20 20
5 8 20 1 13 14 8 9 20 23 5 8 20 4 5 9 19 20 20 23 9 12 16
5 8 20 1 1 1 3 4 5 9 9 14 16 16 18 19 6 15 1 4 12 25 1 3 5 6 14 18 19 1 1 3 6 18 24
Edit: stupid GOG forum .It made the puzzle harder.

well,decoded, now switiching to words 8/
EDIT:Eh, screw it, have some fantasy names:
The sailor city Ycus
Aciedofas entity
saltish bows
Seven more to go!

This is really hard! I'm stuck on these anagrams now (they are anagrams, right?). I must find a line of thought for it. (damn on how much I want master of orion)

That's odd. Most of the lines start of with 5 8 20, so I figured they'd stand for T H E, but line number eight starts off with 5 8 20 5 5 and I don't know of any words that start with a double T.
There's a couple of double of triple numbers in a row, so that narrows down what letters they can be or where the spaces are placed.

Heres a little side note:
O+P+T+I+M+U+S = 15+16+20+9+13+21+19 = 113 is a Prime number; Optimus = Prime

the solitary cyclist
A Case of Identity
The Speckled Band
The Greek Interpreter
The Musgrave Ritual
the Sussex Vampire
The Disappearance of Lady Frances Carfax
The Man with the Twisted Lip
Shoscombe Old Place
His Last Bow
edit: it's out of order, by the way.

The Solitary Cyclist
A Case of Identity
His Last Bow
The Speckled Band
Shoscombe Old Place
the Musgrave Ritual
The Sussex Vampire
The Greek Interpreter
The Man with the Twisted Lip
The Disappearance of Lady Frances Carfax
btw, The Sussex Vampire should really be The Sussex Uampire, because there's a mistake in the riddle, 21 is u not v).
last edit: I've just noticed I had first title wrong (Solarity indstead of Solitary)

Good thing I stopped at the substitution phase. This isn't a Dan Brown universe, I'm no doctor of anagramism (or whatever he called it).

F*ck this! Too much trouble for a free game. The first part is easy (The sailor...), but after that things start to get much complicated. Good luck guys. Who solves this puzzle really deserves a prize.

Ah, so Sherlock Holmes stories. Cool.

my mistake,sorry
edit: looks like we have two winners
Balthier and SLP2000 name your game

Well, I didn't expect to win after I saw Balthier's answer, and I suppose you didn't expect to give away two games, so let's say that being the winner is enough prize for me :)
and I liked the topic very much, as I love Holmes stories.

Well now you deserves a game for being such a nice person. :)

agree