Is there any harm in putting in a massive number into the len parameter of the Read() method?

You cannot read more than 3641144 characters from a stream to a string, as this is the limit for long strings: String Length Limit

From the documentation for the Read() method:

If no len is passed in, ie. 'Read()' then it is up to the Read implementation as to
how much data to return. Some stream classes use this to optimize the amount of
data returned to align this with the underlying storage of the stream.

So you can write like this:
Set inMsg = %request.Content.Read($$$MaxLocalLength)

Macros are defined in %msql.inc:

#define MaxStringLength 3641144
#define MaxLocalLength $zutil(96,39)

See the answers here.

In addition there are a few comments:

• you can use extrinsic function:
  • ExtrinsicFunctions
  • An extrinsic Caché ObjectScript function call operating on a database column
• you can use stored function: Defining and Using Stored Procedures
• you can encrypt the entire database, not just individual fields: Using Encrypted Databases
• you can restrict access to encrypted/sensitive fields to privileged users only: GRANT column-privilege

there are a few comments:

• you can write shorter:
  

  Property CarNumber As %String(TRUNCATE = 1) [ SqlComputeCode = {s {*}=$s(Clear:$$Decrypt({CAR_Number}),1:{CAR_Number})}, SqlComputed, SqlFieldName = CAR_Number ];

• you can make a separate calculated field for the decrypted value, for example:
  

  Property CarNumber As %String(TRUNCATE = 1) [ SqlFieldName = CAR_Number ];
  
  Property CarNumberDecr As %String(TRUNCATE = 1) [ Calculated, SqlComputeCode = {s {*}=$s(Clear:$$Decrypt({CAR_Number}),1:{CAR_Number})}, SqlComputed, SqlFieldName = CAR_NumberDecr ];

• where do you get the value of the Clear variable (see above)?
• you can create your own data type for encrypted strings and override the corresponding methods there: How Data Type Classes Work

  In this case, you will not have to clutter up your queries with unnecessary encryption/decryption operations.

See Deploying code to production site especially the section Limitations for DeployToFile() and InstallFromFile()

There are a couple of points:

• what's wrong with users, roles, privileges, applications, etc.?

  %SYS>do ^SECURITY
   
   
  1) User setup
  2) Role setup
  3) Service setup
  4) Resource setup
  5) Application setup
  6) Auditing setup
  8) SSL configuration setup
  9) Mobile phone service provider setup
  10) OpenAM Identity Services setup
  11) Encryption key setup
  12) System parameter setup
  13) X509 User setup
  14) KMIP server setup
  15) Exit
   
  Option? 12
   
  1) Edit system options
  2) Edit authentication options
  3) Edit LDAP options
  4) Display system options
  5) Export All Security settings
  6) Import All Security settings
  7) Exit
   
  Option? 5
   
  Export ALL security records? Yes => Yes
   
  Warning: Before importing SSL configurations into a different configuration
  the same certificate directories and certificate files must exist in
  the new configuration, otherwise the import will fail.
   
  Export to file name SecurityExport.xml =>
  Parameters? "WNS" =>
  Confirm export of selected security records to SecurityExport.xml? No => yes
   
  Exported x Application security records
  Exported x DocDB security records
  Exported x Event security records
  Exported x KMIPServer security records
  Exported x LDAPConfig security records
  Exported x OpenAMIdentityService security records
  Exported x PhoneProvider security records
  Exported x Resource security records
  Exported x Role security records
  Exported x SQLPrivileges security records
  Exported x SSLConfig security records
  Exported x Service security records
  Exported x System security records
  Exported x User security records
  Exported x X509Credential security records
  Exported x X509Users security records
   
  Export complete
   
  1) Edit system options
  2) Edit authentication options
  3) Edit LDAP options
  4) Display system options
  5) Export All Security settings
  6) Import All Security settings
  7) Exit
   
  Option?

• to transfer JDBC/ODBC settings, you can still use external tools, for example DbVisualizer or SQL Data Lens, where you can choose the data format (CSV, XML, JSON, Excel, etc.), the necessary fields and much more.

Did you copy/paste the query correctly here?

Select Books.nam,Books.print,Books.Relativename,Books.Firstpage,Books.Lastpage,Books.trn,Books.lastissue,
Books.firstissue,Books.person,Books.author,Books.price,Books.cd,Books.ab,Books.pf,Books.ju,Books.er,
Books.qw,Books.qt,Books.mn,
Records.qw,Records.er,Records.ty,Records.ui,Records.op,Records."as",
OrderBooks.mn,OrderBooks.bv,
OrderRecords.sd,OrderRecords.fg,OrderRecords.hj,
Orders.lastdate

From SQLUser.Books Books

INNER JOIN SQLUser.Records Records ON Books.id=Recordsid
INNER JOIN SQLUser.OrderedBooks OrderBooks ON Books.id=OrderBooks.id
INNER JOIN SQLUser.OrderedRecords OrderRecords ON Books.id=OrderRecords
INNER JOIN SQLUser.Orders Orders ON Books.id=Orders.id

where ((Records.qw=OrderBooks.qw) and (Records.er is null))

In addition to tuning tables, I would first look at the query plan and, accordingly, the indexes involved in it.

SFN - system file number

If you select the text with the mouse, you will see spaces on the right in the second "Output".
In general, I think that such details should be enclosed in quotation marks.