NB: biber is needed to (fully) enjoy the following code.
To let BibLaTeX deal with online and print ISBNs, we need register the fields in a datamodel file, the easiest way is to create a <style>.dbx in the working directory.
\ProvidesFile{authortitle.dbx}
\DeclareDatamodelFields[type=field,datatype=literal]{isbnonline}
\DeclareDatamodelFields[type=field,datatype=literal]{isbnprint}
\DeclareDatamodelEntryfields{isbnonline,isbnprint}
Now biber and BibLaTeX know the field, we need to get BibLaTeX to print it in a nice format
\DeclareFieldFormat{isbnonline}{\mkbibacro{ISBN}~(online)\addcolon\space #1}% we need some format ..
\DeclareFieldFormat{isbnprint}{\mkbibacro{ISBN}~(print)\addcolon\space #1}% ... just stole it from standard ISBN
We just patch the bibliography drivers to also print our online and print ISBN.
With the following command patching a driver (that includes \printfield{isbn}) is as easy as \patchmoreisbn{book}.
\newcommand*{\patchmoreisbn}[1]{% let all drivers include the isbn
\xpatchbibdriver{#1}%
{\printfield{isbn}}
{\printfield{isbn}%
\newunit\newblock
\printfield{isbnprint}%
\newunit\newblock
\printfield{isbnonline}}%
{\typeout{patching #1 to include more ISBNs succeded}}%
{\typeout{patching #1 to include more ISBNs failed}}%
}
The full MWE
\documentclass[ngerman, a4paper]{article}
\usepackage[latin1]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{babel}
\usepackage{csquotes}
\usepackage{xpatch}% to patch the editor macros
\usepackage[style=authortitle, backend=biber]{biblatex}
\addbibresource{\jobname.bib}
\begin{filecontents}{authortitle.dbx}
\ProvidesFile{authortitle.dbx}
\DeclareDatamodelFields[type=field,datatype=literal]{isbnonline}
\DeclareDatamodelFields[type=field,datatype=literal]{isbnprint}
\DeclareDatamodelEntryfields{isbnonline,isbnprint}
\end{filecontents}
\begin{filecontents}{\jobname.bib}
@book{AcaryBrogliato201011,
title = {Numerical Methods for Nonsmooth Dynamical Systems},
subtitle = {Applications in Mechanics and Electronics},
titleaddon = {Lecture Notes in Applied and Computational Mechanics},
author = {Vincent Acary and Bernard Brogliato},
publisher = {Springer},
date = {2010},
month = {11},
edition = {1},
isbnonline = {978-3-540-75392-6},
isbnprint = {978-3-642-09464-4},
}
@book{Heuser:Analysis1,
author = {Harro Heuser},
title = {Lehrbuch der Analysis Teil 1},
edition = {17., aktualisierte},
date = {2009},
gender = {sm},
isbn = {978-3-8384-0777-9},
publisher = {Vieweg+Teubner},
location = {Wiesbaden},
}
\end{filecontents}
\DeclareFieldFormat{isbnonline}{\mkbibacro{ISBN}~(online)\addcolon\space #1}% we need some format ..
\DeclareFieldFormat{isbnprint}{\mkbibacro{ISBN}~(print)\addcolon\space #1}% ... just stole it from standard ISBNa
\newcommand*{\patchmoreisbn}[1]{% let all drivers include the isbn
\xpatchbibdriver{#1}%
{\printfield{isbn}}
{\printfield{isbn}%
\newunit\newblock
\printfield{isbnprint}%
\newunit\newblock
\printfield{isbnonline}}%
{\typeout{patching #1 to include more ISBNs succeded}}%
{\typeout{patching #1 to include more ISBNs failed}}%
}
% patch all drivers
\patchmoreisbn{article}
\patchmoreisbn{book}
\patchmoreisbn{collection}
\patchmoreisbn{inbook}
\patchmoreisbn{incollection}
\patchmoreisbn{inproceedings}
\patchmoreisbn{manual}
\patchmoreisbn{periodical}
\patchmoreisbn{proceedings}
\patchmoreisbn{report}
\patchmoreisbn{thesis}
\begin{document}
\nocite{*}
\printbibliography
\end{document}
produces

isbnfield is of the literal type so you can do list processing with\forcsvfield. You can also extend the default data model to makeisbnof the list type. There was some discussion of this at the github repo. – Audrey Mar 21 '13 at 19:29