The BTL now has a project page at Bioinformatics.org. Whilst we make the transition to Bioinformatics.org, please see the main BTL website for information. The current release is still Version 3.0, and is available for download via either site. In the coming weeks we aim to update the BTL code with a number of bug fixes and better support for recent compilers. The documentation and regression tests will also be improved.
Also, have a look at the original BTL paper (this link requires registration to the online version of Bioinformatics)
Ref: Bioinformatics, Vol. 17, no. 8, 2001, Pages 729-737.
The BTL team, 2002