I read that you can make a sumti from a selbri by using "le"
i.e., "le <selbri>" is a sumti meaning "something (that I have in
mind) which fits in the first place of <selbri>. How general is this
procedure of making sumti? In particular, can I convert partial bridi
into sumti using some similar construction?
For example, I can say "mi tcidu le cukta" for "I read a book." What
if wanted to say "I read a book (written) by Tolkien?" In this case,
if will be useful to be able to convert the partial bridi "cukta fi la
tolkin" to something like "le(?) cukta fi la tolkin." I don't think
the usage of "le" here is right but what I wanted to say is "something
that fits in the first place of 'cukta fi la tolkin'.
You're almost right. The problem is that with "mi tcidu le cukta fi la tolkin" the fi is attaching {la tolkin} to tcidu, not cukta. You need {be} to link it to the sumti at hand: "mi tcidu le cukta be fi la tolkin".
On a related note, is saying "mi tcidu le cukta pe la tolkin" a good
way to say "I read a book by Tolkien"
"I read a book associated with Tolkein."
So, it could be a book by Tolkein, or about Tolkein, or ...