Why do books say "ספר" on them?

Question

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.

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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

Answer 1

When possible, I look for the first place in Tana"ch that the Hebrew word is used (assuming, of course, that it is a Biblically used word.

"Sefer" is mentioned "early" in the Torah in Breishit 5:1, "This is the 'sefer' of the generations of Adam (man - humanity)" - My loose translation esp. of the word "Adam", in the is context.

I linked to the Mikra'ot Gedolot version, intentionally, because each commentary has a slightly different explanation of the word 'sefer'.

Seforno (I have a feeling that his name originated from 'sefer', BTW, though I don't know what the connection is.) translates it as "story".

Rash"i translates this as "accounting" or "counting" - i.e. connected to numerology.

In terms of using "sefer" for the names of most books, either of these terms may apply. Most works called "sefer" has some form of "accounting". Sefer Minyan Hamitzvot (from Ramba"m) is an example. The mitzvoth are counted / assigned a number. Why not just leave the word "sefer" out? Perhaps, but, maybe for someone who didn't know what the book was about, isn't it nice to know that by saying "sefer", now I know this book has some sort of "counting" or a method of assigning a number to the ideas in the book?

The 2nd definition - "story" could also mean "lessons". I.e. - the "stories" being told are not like a bedtime story where it's just nice to hear. These are meant to be examples, morals, or lessons taught on how to behave and conduct your life properly. Sefer Chafetz Chaim has lessons and examples / stories (a few) to teach you individual lessons on how to conduct your life.

Incorporating from a comment - if it were just called "Chafetz Chaim" you would have some general idea that this book had something related about "desiring life" or improving how you live. By saying "sefer", you have some idea of the structure - it's telling me some stories.

(Note - Rav Kagan is called "The Chafetz Chayim" because it was custom to name people after one of their most notable seforim that they wrote, not the other way around. The book is not named after the Rav!)

Granted, I'm sure that my explanation may not apply to every book with the title "Sefer", but, probably a majority. If you think of any that may break this rule, I'd like to know. But, IMO, that may be one of several exceptions to the trend.