Why do Jewish books often say "ספר" on the spine, cover, or cover page?
The word is synonymous with "book" in modern Hebrew, and some closely related concept like "document" or "scroll" in Tana"ch. I find it strange for book titles (or at least the covers that contain them) to have it in them due to redundancy. The numbered hypotheses below are ways I have speculated to mitigate the strangeness, but there are reasons listed explaining my scepticism about each one.
Not included is the absurd possibility that "ספר" is intended as a helpful descriptor for a person holding the object in their hand to identify it by.
1. The null hypothesis
There is no good reason behind inclusion or exclusion of the word and this decision is simply subject to the writer's or printer's whim. The only way to disconfirm or disprove this argument would be to gather a large volume of indicative evidence, to which I might have access but not the tools to analyze.
2. Association with sanctity
The author is going for the connotation of sanctity through association with the "classical" meaning of the word. It is used in several places to refer exclusively to biblical books - e.g. in this mishna. Perhaps that exclusivity has broader application than "Scripture" and people are implying that their works belong in that expanded category, but I haven't seen anyone address this obviously in an introduction.
3. Emphasize length/genre
Like "book", the word could name a self-contained, long, factual unit of printing. As such, people might wish to specify that right on the cover as opposed to being one of. . .
- commentary; {counterexample}
- responsa; {counterexample 1, counterexample 2}
- compilation; {counterexample 1 ironic counterexample 2}
- pamphlet; {?}
4. Differentiate from generic phrase
Less extreme than the absurd possibility mentioned above, if a book has a generic title like this one, it could be useful to differentiate it from the concept the title names, especially when referred to without context. This does not pragmatically apply to most instances.
In some cases the appendage of the word "ספר" varies by printing of a given work, but in others it seems fossilized such that it appears in each printing I encounter. This phenomenon is less explicable in my opinion than the occurrence of the word in general (not specifically Torah-related) contexts, which tend to have obvious reasons for being there, such as fitting naturally into a title which would otherwise be just a topic, as @Yishai mentioned in a comment.
An acceptable answer will make a convincing argument based on a lot of observations and patterns induced from them. An "Accept"able answer will invoke express wisdom of printers or publishers (or the people who study them) of many books.
Some relevant examples in response to comments:
- Igros Moshe: first edition (I believe) from 1959, includes "ספר"
- Sh'nei Luchos Hab'ris: commonly abbreviated, includes "ספר"
- Halachos K'tanos: both
