Colloms - High Performance Loudspeakers (Pentech Press - but may have changed publisher since I bought my early copy).
Dickason - Loudspeaker Design Cookboook (Old Colony).
Speaker Builder magazine was - for some time - a quite informative source (it's the sort of knowledge you accumulate over the years). But it got subsumed back into Audio Electronics and I'm not sure what their status is now (ads I see for Old Colony haven't changed in years - which I always take to be an indicator of moribundity).
I looked at the latest edition of Colloms. It's 60 quid! Mind you I suppose as a reference work for those more knowledgeable than I, that probably isn't too bad. There are also older ones out there, I might try Edition 5.