Andrew Gray & G. B. Mathews 
Treatise on Bessel Functions [PDF ebook] 
And Their Applications to Physics

Whilst the greatest effort has been made to ensure the quality of this text, due to the historical nature of this content, in some rare cases there may be minor issues with legibility. Some readers may be inclined to think that the earlier chapters contain a needless amount of tedious analysis; but it must be remembered that the properties of the Bessel functions are not without an interest of their own on purely mathematical grounds, and that they afford excellent illustrations of the more recent theory of differential equations, and of the theory of a complex variable. And even from the purely physical point of View it is impossible to say that an analytical formula is useless for practical purposes; it may be so now, but experience has repeatedly shown that the most abstract analysis may unexpectedly prove to be of the highest importance in mathematical physics. As a matter of fact it will be found that little, if any, of the analytical theory included in the present work has failed to be of some use or other in the later chapters; and we are so far from thinking that anything superfluous has been inserted, that We could almost wish that space would have allowed of a more extended treatment, especially in the chapters on the complex theory and on definite integrals.