Offbeat investing can be Captain Marvelous; The Almanac of Investments, Edited by Alan Crittenden, Novato, Calif: Crittenden Books. 514 pp. $24.95.

Go up to someone on the street and ask him what he thinks when you say ''investments.'' Unless he just looks at you strangely and walks away, he'll most likely answer ''stocks'' or ''bonds.''

Not many of those who stayed around to answer would mention 45 r.p.m. records , furniture, books, cows, or comic books. Yet, as the accompanying table shows, people who knew about these investments and put their money in the right ones might have done better with Captain Marvel and the Partridge Family than with many stocks and bonds.

If you want to find out something about these or any investments, whether it's diamonds, cattle, mutual funds, art, furniture, or commodities, you have to buy a whole book on the subject, maybe two. After some reading, you may discover that the investment doesn't interest you or the potential rewards aren't worth the risk. If you try to save some money by checking a book out of the library, it's apt to be out of date.

When you've built up a library of books you can't use, you might wish someone would publish a book to tell you just enough - but not too much - about nearly all the investment alternatives.

This is what Mr. Crittenden, a San Francisco publisher of investment newsletters, has provided. He has enlisted the help of several other experts to write most of the chapters.

In an effort to pack as many investments between two covers as he can, Mr. Crittenden has allowed no more than a few pages to each one. Oriental rugs, for example, despite thousands of years of history, hundreds of varieties, and many factors that can affect their value, get just 51/2 pages. This includes one photograph - not of a rug, but Richard Nixon with the Shah of Iran, to remind us how prices can change when major events occur. When the Shah's regime fell in 1979, so did the price of Oriental rugs.

The few pages devoted to each subject give us some history behind the investment, information on today's market, and investment pros and cons. We also get some tips on how to choose an item, the minimum investment you can expect, possible tax consequences, and a few little-known facts (while ''rug'' and ''carpet'' are used interchangeably, a rug is 6 by 9 feet or smaller; a carpet is larger).

Most entries also have a short bibliography, in case you do want to wade through a book or two on the subject.

There are also very useful subheadings on ''ripoffs and scandals'' which should be read by anyone seriously thinking of an investment, particularly if it comes from someone he or she doesn't know. For example, there have been a few instances of people buying counterfeit comic books.

Although there are advantages to brevity, it can be carried too far. The last chapter, for example, tries to tell us ''How to Plan an Investment Strategy'' in less than four pages. There is advice here about setting goals, establishing time lines to reach those goals, and sticking to both, but a little more on how to go about this would help.

None of the entries in the Almanac provide all the information you need to invest your money. If an investment is new to you, you still need to do extra reading and talk to experts who specialize in the field. But if you want more some basic information on the alternatives that take the term ''investment'' beyond stocks and bonds, to include stamps, records, real estate, cattle, art or antiques, this will be a useful book - a good investment. Comparing stocks with Monets

What are the best and worst investments? In ''The Almanac of Investments,'' Alan Crittenden provides a list of popular - and not-so-popular - investments and shows how their value changed between the start of 1980 and mid-1983. Here are some of the items from that list: Investment Percentage change 1980 to '83 1. Planet Comics (set) +106% 2. Hambrecht & Quist index +77% (technology stocks) 3. Action Comics (set) +73% 4. Fixed-income funds +45% 5. ''I Think I Love You'' +45% (Partridge Family, 45 r.p.m. record) 6. Thoroughbred horses (average prices) +39% 7. Growth-and-income funds +35% 8. Capital-appreciation funds +32% 9. American furniture +28% 10. Standard & Poor's 500 +24% 11. Dow Jones industrial average +22% 12. Average price, new single-family home +17% 13. Chinese ceramics -1% 14. Municipal bond yield averages -7% 15. ''Moonrise, Hernandez, New Mexico'' -11% (Ansel Adams photograph) 16. 19th-century European paintings -11% 17. Industrial bonds (average annual yields) -15% 18. Old-master paintings -15% 19. Gold (cash prices, troy oz.) -28% Handy & Harmon base price 20. Diamonds, 1 carat, D flawless -65%