In the pre-computer days, we used to have school handouts made on ditto machines or spirit duplicators that printed in purple ink. Once photocopiers became more common, we sometimes needed to make photocopies of these pale materials. One trick was to insert the sheet with the purple printing into a translucent yellow report cover, so that the ink appeared as black text. In this way, you could make a black-and-white photocopy with darker printing.
I downloaded the image and experimented a bit with Irfanview. On the "Image" menu there is a setting to turn the image to a Negative, which sometimes works to make writing more clear. However, doing this rendered the image nearly unreadable, so I went back to the original and started over.
Playing around with the different color channels, I got the writing to turn black on a greenish background. Having black text might make the letter easier to read.
Perhaps with Photoshop, you could use these techniques and others to simulate the process of overlaying the document with a color which is across the color wheel from the ink color, similar to what I used to do with the purple-and-yellow trick.
Cutting a photocopy into strips, or masking it so that only one line appears at a time, may make it easier to focus on just that line. Blowing it up may help also.
For the handwriting itself, there is an online tutorial at the UK National Archives. The crossed lines might be easier to read, if you could find documents in the same style of handwriting which are not crossed, and practice first on those. Try to find or make a letter chart to use for reference. You have the opening "Vittoria 29th June 1813" and "Dear" (perhaps "Dear Sirs"?) to start off with.
We can give you general tips here, but it's not practical to write answers which are transcriptions of an entire multi-page document. See the related question Reading a Will to Get Land Information and other questions marked palaeography for other ideas.
One thing you might try is finding printed accounts of the Battle of Vittoria, from sources like Google Books or the British Newspaper Archives, that might give you a feeling for the vocabulary that might be used in an account like this.
As you need help reading specific lines, feel free to post new questions asking for help with line-readings.
Basic tips on reading old handwriting