It sounds like your question is whether your digital recipe library was a "work made for hire" and therefore the property of your employer.
The first thing you should check is the language of your written employment agreement, if any, which may obligate you to assign any inventions/creations/recipes to your employer.
Barring explicit mention in your employment agreement, this may become a common law issue. The Restatement (Second) of Agency defines a work made for hire as a work made by an employee within the scope of his or her employment. It doesn't sound like you are disputing that you were an employee. The question becomes, then, whether the creation of this library was "within the scope" of your employment. The common law test for scope is whether the activity (a) is of the kind the employee is employed to perform, (b) occurs substantially within authorized time and space limits, and (c) be actuated, at least in part, by a purpose serving the employer.
Presumably, you worked on this digital library while at work, and your work on it was done, at least in part, to serve your employer. So you're left with the question of whether work on a digital recipe library is "of the kind" of work that you were hired to do. Sounds like a pretty gray area. Were you just hired to cook food, or also to organize and compile recipes? If your employer really wants to take your library sales profits from you, they may argue that organizing recipes was one of the things you were hired to do for them, one of the ancillary parts of the job, just like washing the occasional dish or sharpening the occasional knife.
And, of course, all of the above is only the musings of a person who has but a facile grasp of the law, and should not be relied upon. If you want legal advice, you have to consult a lawyer.